drm/ttm: fix adding foreign BOs to the swap LRU

BugLink: http://bugs.launchpad.net/bugs/1546572

It doesn't make any sense to try to swap out imported BOs.
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit ed704a43)
Signed-off-by: Alberto Milone <alberto.milone@canonical.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>

drivers/gpu/drm/ttm/ttm_bo.c

@@ -176,7 +176,7 @@ void ttm_bo_add_to_lru(struct ttm_buffer_object *bo)
 		list_add_tail(&bo->lru, &man->lru);
 		kref_get(&bo->list_kref);
 
-		if (bo->ttm != NULL) {
+		if (bo->ttm && !(bo->ttm->page_flags & TTM_PAGE_FLAG_SG)) {
 			list_add_tail(&bo->swap, &bo->glob->swap_lru);
 			kref_get(&bo->list_kref);
 		}