drm/amdgpu: fix dropped backing store handling in amdgpu_dma_buf_move_notify

bo->tbo.resource can now be NULL.

Signed-off-by: Christian König <christian.koenig@amd.com>
Bug: https://gitlab.freedesktop.org/drm/amd/-/issues/1811
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211210083927.1754-1-christian.koenig@amd.com

Author: Christian König

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

@@ -384,7 +384,7 @@ amdgpu_dma_buf_move_notify(struct dma_buf_attachment *attach)
 	struct amdgpu_vm_bo_base *bo_base;
 	int r;
 
-	if (bo->tbo.resource->mem_type == TTM_PL_SYSTEM)
+	if (!bo->tbo.resource || bo->tbo.resource->mem_type == TTM_PL_SYSTEM)
 		return;
 
 	r = ttm_bo_validate(&bo->tbo, &placement, &ctx);