drm: Restore the NULL check for drm_gem_object_put()

Some users want to pass NULL to drm_gem_object_put(), but those using
__drm_gem_object_put() did not. Compromise, have both and let the
compiler sort it out.

drm_gem_fb_destroy() calls drm_gem_object_put() with NULL obj causing:
[ 11.584209] BUG: kernel NULL pointer dereference, address: 0000000000000000
[ 11.584213] #PF: supervisor write access in kernel mode
[ 11.584215] #PF: error_code(0x0002) - not-present page
[ 11.584216] PGD 0 P4D 0
[ 11.584220] Oops: 0002 [#1] SMP NOPTI
[ 11.584223] CPU: 7 PID: 1571 Comm: gnome-shell Tainted: G E 5.7.0-rc1-1-default+ #27
[ 11.584225] Hardware name: Micro-Star International Co., Ltd. MS-7A31/X370 XPOWER GAMING TITANIUM (MS-7A31), BIOS 1.MR 12/03/2019
[ 11.584237] RIP: 0010:drm_gem_fb_destroy+0x28/0x70 [drm_kms_helper]
<snip>
[ 11.584256] Call Trace:
[ 11.584279] drm_mode_rmfb+0x189/0x1c0 [drm]
[ 11.584299] ? drm_mode_rmfb+0x1c0/0x1c0 [drm]
[ 11.584314] drm_ioctl_kernel+0xaa/0xf0 [drm]
[ 11.584329] drm_ioctl+0x1ff/0x3b0 [drm]
[ 11.584347] ? drm_mode_rmfb+0x1c0/0x1c0 [drm]
[ 11.584421] amdgpu_drm_ioctl+0x49/0x80 [amdgpu]
[ 11.584427] ksys_ioctl+0x87/0xc0
[ 11.584430] __x64_sys_ioctl+0x16/0x20
[ 11.584434] do_syscall_64+0x5f/0x240
[ 11.584438] entry_SYSCALL_64_after_hwframe+0x44/0xa9
[ 11.584440] RIP: 0033:0x7f0ef80f7227

Reported-by: Nirmoy Das <[e-mail]>
Fixes: b5d250744ccc ("drm/gem: fold drm_gem_object_put_unlocked and __drm_gem_object_put()")
Signed-off-by: Chris Wilson <[e-mail]>
Cc: Nirmoy Das <[e-mail]>
Cc: Emil Velikov <[e-mail]>
Cc: Christian König <[e-mail]>.
Acked-by: Nirmoy Das <[e-mail]>
Reviewed-by: Emil Velikov <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/i915/gem/i915_gem_object.h

···
 static inline void
 i915_gem_object_put(struct drm_i915_gem_object *obj)
 {
-	drm_gem_object_put(&obj->base);
+	__drm_gem_object_put(&obj->base);
 }
 
 #define assert_object_held(obj) dma_resv_assert_held((obj)->base.resv)

include/drm/drm_gem.h

···
 	kref_get(&obj->refcount);
 }
 
+__attribute__((nonnull))
+static inline void
+__drm_gem_object_put(struct drm_gem_object *obj)
+{
+	kref_put(&obj->refcount, drm_gem_object_free);
+}
+
 /**
  * drm_gem_object_put - drop a GEM buffer object reference
  * @obj: GEM buffer object
···
 static inline void
 drm_gem_object_put(struct drm_gem_object *obj)
 {
-	kref_put(&obj->refcount, drm_gem_object_free);
+	if (obj)
+		__drm_gem_object_put(obj);
 }
 
 void drm_gem_object_put_locked(struct drm_gem_object *obj);