drm/ttm: fix warning after moving resource to ghost obj

After we moved the resource to the ghost the bo->resource pointer needs
to be reset since the owner of the resource is now the ghost.
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210607175737.1405-1-christian.koenig@amd.com

drivers/gpu/drm/ttm/ttm_bo_util.c

@@ -507,6 +507,7 @@ static int ttm_bo_move_to_ghost(struct ttm_buffer_object *bo,
 		ghost_obj->ttm = NULL;
 	else
 		bo->ttm = NULL;
+	bo->resource = NULL;
 
 	dma_resv_unlock(&ghost_obj->base._resv);
 	ttm_bo_put(ghost_obj);