backends/drm: don't update outputs while the session isn't active

BUG: 468715

src/backends/drm/drm_gpu.cpp

@@ -218,6 +218,9 @@ void DrmGpu::initDrmResources()
 
 bool DrmGpu::updateOutputs()
 {
+    if (!m_isActive) {
+        return false;
+    }
     waitIdle();
     DrmUniquePtr<drmModeRes> resources(drmModeGetResources(m_fd));
     if (!resources) {