backends/drm: attempt a modeset on output disabling

When modesets are necessary, they are attempted when an output on the given
GPU gets presented. With multi-gpu setups however, the situation can arise
where there is only one disabled output on a GPU; in that case KWin eternally
waits and never properly turns off the display.
In order to work around this, explicitly call DrmGpu::maybeModeset when
an output gets disabled.

BUG: 449878
FIXED-IN: 5.24.4

src/backends/drm/drm_output.cpp

@@ -394,6 +394,9 @@ void DrmOutput::applyQueuedChanges(const WaylandOutputConfig &config)
 
     auto props = config.constChangeSet(this);
     setEnabled(props->enabled && m_pipeline->pending.crtc);
+    if (!isEnabled() && m_pipeline->needsModeset()) {
+        m_gpu->maybeModeset();
+    }
     moveTo(props->pos);
     setScale(props->scale);
     setTransformInternal(props->transform);