drm/amdgpu: add missing NULL check

bo->tbo.resource can easily be NULL here.

Link: https://gitlab.freedesktop.org/drm/amd/-/issues/2902
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
CC: stable@vger.kernel.org

Author: ChristianKoenigAMD

drivers/gpu/drm/amd/amdgpu/amdgpu_object.h

@@ -252,7 +252,7 @@ static inline bool amdgpu_bo_in_cpu_visible_vram(struct amdgpu_bo *bo)
 	struct amdgpu_device *adev = amdgpu_ttm_adev(bo->tbo.bdev);
 	struct amdgpu_res_cursor cursor;
 
-	if (bo->tbo.resource->mem_type != TTM_PL_VRAM)
+	if (!bo->tbo.resource || bo->tbo.resource->mem_type != TTM_PL_VRAM)
 		return false;
 
 	amdgpu_res_first(bo->tbo.resource, 0, amdgpu_bo_size(bo), &cursor);