backend/drm: allocate a CRTC in drm_connector_test

We can't perform a test-only atomic commit if the connector is
missing a CRTC.

backend/drm/drm.c

@@ -414,6 +414,8 @@ static bool test_buffer(struct wlr_drm_connector *conn,
 	return drm_crtc_commit(conn, &conn->output.pending, DRM_MODE_ATOMIC_TEST_ONLY);
 }
 
+static bool drm_connector_alloc_crtc(struct wlr_drm_connector *conn);
+
 static bool drm_connector_test(struct wlr_output *output) {
 	struct wlr_drm_connector *conn = get_drm_connector_from_output(output);
 
@@ -427,6 +429,14 @@ static bool drm_connector_test(struct wlr_output *output) {
 		}
 	}
 
+	if (drm_connector_state_active(conn, &output->pending)) {
+		if (!drm_connector_alloc_crtc(conn)) {
+			wlr_drm_conn_log(conn, WLR_DEBUG,
+				"No CRTC available for this connector");
+			return false;
+		}
+	}
+
 	if ((output->pending.committed & WLR_OUTPUT_STATE_BUFFER) &&
 			output->pending.buffer_type == WLR_OUTPUT_STATE_BUFFER_SCANOUT) {
 		if (!test_buffer(conn, output->pending.buffer)) {