backend/drm: use panel_orientation if it is set

This fixes an issue where the panel_orientation parameter was set but wlroots was ignoring it and
returning OUTPUT_TRANSFORM_NORMAL.

Fixes 2e12de96

backend/drm/drm.c

@@ -1096,7 +1096,7 @@ uint32_t wlr_drm_connector_get_id(struct wlr_output *output) {
 enum wl_output_transform wlr_drm_connector_get_panel_orientation(
 		struct wlr_output *output) {
 	struct wlr_drm_connector *conn = get_drm_connector_from_output(output);
-	if (conn->props.panel_orientation) {
+	if (!conn->props.panel_orientation) {
 		return WL_OUTPUT_TRANSFORM_NORMAL;
 	}