drm/udl: properly check for error pointers

The drm_prime_pages_to_sg() function never returns NULL pointers, only
error pointers and valid pointers.
Signed-off-by: Haixia Shi <hshi@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/udl/udl_dmabuf.c

@@ -99,9 +99,9 @@ static struct sg_table *udl_map_dma_buf(struct dma_buf_attachment *attach,
 
 	page_count = obj->base.size / PAGE_SIZE;
 	obj->sg = drm_prime_pages_to_sg(obj->pages, page_count);
-	if (!obj->sg) {
-		DRM_ERROR("sg is null.\n");
-		return ERR_PTR(-ENOMEM);
+	if (IS_ERR(obj->sg)) {
+		DRM_ERROR("failed to allocate sgt.\n");
+		return ERR_CAST(obj->sg);
 	}
 
 	sgt = &udl_attach->sgt;