drm_gpu: don't bother trying to assign a crtc to a connector that isn't connected

BUG: 457002

src/backends/drm/drm_gpu.cpp

@@ -356,7 +356,7 @@ DrmPipeline::Error DrmGpu::checkCrtcAssignment(QVector<DrmConnector *> connector
     }
     auto connector = connectors.takeFirst();
     auto pipeline = connector->pipeline();
-    if (!pipeline->enabled()) {
+    if (!pipeline->enabled() || !connector->isConnected()) {
         // disabled pipelines don't need CRTCs
         pipeline->setCrtc(nullptr);
         return checkCrtcAssignment(connectors, crtcs);