drm/amdgpu: Add a NULL check for freeing root PT

This patch adds a NULL check to fix this crash reported during the
freeing of root PT entry:

BUG: unable to handle page fault for address: ffffc9002d637aa0
#PF: supervisor write access in kernel mode
#PF: error_code(0x0002) - not-present page
RIP: 0010:amdgpu_vm_pt_free+0x66/0xe0 [amdgpu]
PKRU: 55555554
Call Trace:
<TASK>
amdgpu_vm_pt_free_root+0x60/0xa0 [amdgpu]
amdgpu_vm_fini+0x2cb/0x5d0 [amdgpu]
? amdgpu_ctx_mgr_entity_fini+0x53/0x1c0 [amdgpu]
amdgpu_driver_postclose_kms+0x191/0x2d0 [amdgpu]
drm_file_free.part.0+0x1e5/0x260 [drm]

Cc: Christian König <[e-mail]>
Cc: Alex Deucher <[e-mail]>
Cc: Felix Kuehling <[e-mail]>
Cc: Rajneesh Bhardwaj <[e-mail]>
Acked-by: Christian König <[e-mail]>
Signed-off-by: Shashank Sharma <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/amd/amdgpu/amdgpu_vm_pt.c

···
 	struct amdgpu_vm_pt_cursor cursor;
 	struct amdgpu_vm_bo_base *entry;
 
-	for_each_amdgpu_vm_pt_dfs_safe(adev, vm, NULL, cursor, entry)
-		amdgpu_vm_pt_free(entry);
+	for_each_amdgpu_vm_pt_dfs_safe(adev, vm, NULL, cursor, entry) {
+		if (entry)
+			amdgpu_vm_pt_free(entry);
+	}
 }
 
 /**