drm/ttm: Skip swapout if ttm object is not populated

Swapping a ttm object which has no backend pages makes no sense.
Suggested-by: Christian König <christian.koenig@amd.com>
Signed-off-by: xinhui pan <xinhui.pan@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210521083112.33176-1-xinhui.pan@amd.com
CC: stable@kernel.org
Signed-off-by: Christian König <christian.koenig@amd.com>

drivers/gpu/drm/ttm/ttm_device.c

@@ -145,7 +145,7 @@ int ttm_device_swapout(struct ttm_device *bdev, struct ttm_operation_ctx *ctx,
 			list_for_each_entry(bo, &man->lru[j], lru) {
 				uint32_t num_pages;
 
-				if (!bo->ttm ||
+				if (!bo->ttm || !ttm_tt_is_populated(bo->ttm) ||
 				    bo->ttm->page_flags & TTM_PAGE_FLAG_SG ||
 				    bo->ttm->page_flags & TTM_PAGE_FLAG_SWAPPED)
 					continue;