drm/ttm: tt destroy move null check to outer function.

This just makes things easier later.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200915024007.67163-4-airlied@gmail.com

drivers/gpu/drm/ttm/ttm_bo.c

@@ -1613,6 +1613,8 @@ EXPORT_SYMBOL(ttm_bo_swapout_all);
 
 void ttm_bo_tt_destroy(struct ttm_buffer_object *bo)
 {
+	if (bo->ttm == NULL)
+		return;
 	ttm_tt_destroy(bo->bdev, bo->ttm);
 	bo->ttm = NULL;
 }