backend/drm: stop checking for wlr_client_buffer

wlr_client_buffer is a bad indicator for direct scan-out.
Compositors might use the underlying wlr_buffer instead.

backend/drm/drm.c

@@ -1690,8 +1690,7 @@ static void handle_page_flip(int fd, unsigned seq,
 	 * data between the GPUs, even if we were using the direct scanout
 	 * interface.
 	 */
-	if (!drm->parent && plane->current_fb &&
-			wlr_client_buffer_get(plane->current_fb->wlr_buf)) {
+	if (!drm->parent) {
 		present_flags |= WLR_OUTPUT_PRESENT_ZERO_COPY;
 	}