backend/drm: Ensure a primary fb is available when configuring an output

backend/drm/drm.c

@@ -684,6 +684,12 @@ static bool drm_connector_test(struct wlr_output *output,
 	struct wlr_drm_connector_state pending = {0};
 	drm_connector_state_init(&pending, conn, state);
 
+	if (state->allow_reconfiguration && !pending.primary_fb) {
+		wlr_drm_conn_log(conn, WLR_DEBUG,
+			"No primary frame buffer available for this connector");
+		goto out;
+	}
+
 	if ((state->committed & WLR_OUTPUT_STATE_ADAPTIVE_SYNC_ENABLED) &&
 			state->adaptive_sync_enabled &&
 			!drm_connector_supports_vrr(conn)) {