drm/xe: Add config control for svm flush work

Without CONFIG_DRM_XE_GPUSVM set, GPU SVM is not initialized thus below
warning pops. Refine the flush work code to be controlled by the config
to avoid below warning:
"
[ 453.132028] ------------[ cut here ]------------
[ 453.132527] WARNING: CPU: 9 PID: 4491 at kernel/workqueue.c:4205 __flush_work+0x379/0x3a0
[ 453.133355] Modules linked in: xe drm_ttm_helper ttm gpu_sched drm_buddy drm_suballoc_helper drm_gpuvm drm_exec
[ 453.134352] CPU: 9 UID: 0 PID: 4491 Comm: xe_exec_mix_mod Tainted: G U W 6.15.0-rc3+ #7 PREEMPT(full)
[ 453.135405] Tainted: [U]=USER, [W]=WARN
...
[ 453.136921] RIP: 0010:__flush_work+0x379/0x3a0
[ 453.137417] Code: 8b 45 00 48 8b 55 08 89 c7 48 c1 e8 04 83 e7 08 83 e0 0f 83 cf 02 89 c6 48 0f ba 6d 00 03 e9 d5 fe ff ff 0f 0b e9 db fd ff ff <0f> 0b 45 31 e4 e9 d1 fd ff ff 0f 0b e9 03 ff ff ff 0f 0b e9 d6 fe
[ 453.139250] RSP: 0018:ffffc90000c67b18 EFLAGS: 00010246
[ 453.139782] RAX: 0000000000000000 RBX: ffff888108a24000 RCX: 0000000000002000
[ 453.140521] RDX: 0000000000000001 RSI: 0000000000000000 RDI: ffff8881016d61c8
[ 453.141253] RBP: ffff8881016d61c8 R08: 0000000000000000 R09: 0000000000000000
[ 453.141985] R10: 0000000000000000 R11: 0000000008a24000 R12: 0000000000000001
[ 453.142709] R13: 0000000000000002 R14: 0000000000000000 R15: ffff888107db8c00
[ 453.143450] FS: 00007f44853d4c80(0000) GS:ffff8882f469b000(0000) knlGS:0000000000000000
[ 453.144276] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 453.144853] CR2: 00007f4487629228 CR3: 00000001016aa000 CR4: 00000000000406f0
[ 453.145594] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 453.146320] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[ 453.147061] Call Trace:
[ 453.147336] <TASK>
[ 453.147579] ? tick_nohz_tick_stopped+0xd/0x30
[ 453.148067] ? xas_load+0x9/0xb0
[ 453.148435] ? xa_load+0x6f/0xb0
[ 453.148781] __xe_vm_bind_ioctl+0xbd5/0x1500 [xe]
[ 453.149338] ? dev_printk_emit+0x48/0x70
[ 453.149762] ? _dev_printk+0x57/0x80
[ 453.150148] ? drm_ioctl+0x17c/0x440
[ 453.150544] ? __drm_dev_vprintk+0x36/0x90
[ 453.150983] ? __pfx_xe_vm_bind_ioctl+0x10/0x10 [xe]
[ 453.151575] ? drm_ioctl_kernel+0x9f/0xf0
[ 453.151998] ? __pfx_xe_vm_bind_ioctl+0x10/0x10 [xe]
[ 453.152560] drm_ioctl_kernel+0x9f/0xf0
[ 453.152968] drm_ioctl+0x20f/0x440
[ 453.153332] ? __pfx_xe_vm_bind_ioctl+0x10/0x10 [xe]
[ 453.153893] ? ioctl_has_perm.constprop.0.isra.0+0xae/0x100
[ 453.154489] ? memory_bm_test_bit+0x5/0x60
[ 453.154935] xe_drm_ioctl+0x47/0x70 [xe]
[ 453.155419] __x64_sys_ioctl+0x8d/0xc0
[ 453.155824] do_syscall_64+0x47/0x110
[ 453.156228] entry_SYSCALL_64_after_hwframe+0x76/0x7e
"

v2 (Matt):
refine commit message to have more details
add Fixes tag
move the code to xe_svm.h which already have the config
remove a blank line per codestyle suggestion

Fixes: 63f6e480d115 ("drm/xe: Add SVM garbage collector")
Cc: Matthew Brost <[e-mail]>
Signed-off-by: Shuicheng Lin <[e-mail]>
Reviewed-by: Matthew Brost <[e-mail]>
Signed-off-by: Matthew Brost <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

drivers/gpu/drm/xe/xe_svm.c

···
 }
 #endif
 
-
 /**
  * xe_svm_handle_pagefault() - SVM handle page fault
  * @vm: The VM.
···
 	return 0;
 }
 #endif
+
+/**
+ * xe_svm_flush() - SVM flush
+ * @vm: The VM.
+ *
+ * Flush all SVM actions.
+ */
+void xe_svm_flush(struct xe_vm *vm)
+{
+	if (xe_vm_in_fault_mode(vm))
+		flush_work(&vm->svm.garbage_collector.work);
+}

drivers/gpu/drm/xe/xe_svm.h

···
 #define xe_svm_notifier_unlock(vm__)	\
 	drm_gpusvm_notifier_unlock(&(vm__)->svm.gpusvm)
 
+void xe_svm_flush(struct xe_vm *vm);
+
 #else
 #include <linux/interval_tree.h>
 
···
 }
 
 static inline void xe_svm_notifier_unlock(struct xe_vm *vm)
+{
+}
+
+static inline void xe_svm_flush(struct xe_vm *vm)
 {
 }
 #endif

drivers/gpu/drm/xe/xe_vm.c

···
 	}
 
 	/* Ensure all UNMAPs visible */
-	if (xe_vm_in_fault_mode(vm))
-		flush_work(&vm->svm.garbage_collector.work);
+	xe_svm_flush(vm);
 
 	err = down_write_killable(&vm->lock);
 	if (err)