monitor-manager-kms: Disable CRTCs if there is no monitor

After last monitor gets unplugged from the system, hotplug detection may
no longer work on Intel GFX.

This is because we didn't trigger a modeset to disable CRTCs, and i915
requires it to make hotplug detection continue to work [1].

Ensure disabled CRTCs are unset and post a modeset to disable them.

[1] https://www.kernel.org/doc/html/latest/gpu/i915.html#hotplug

https://gitlab.freedesktop.org/drm/intel/-/issues/2602

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1561>

src/backends/native/meta-renderer-native.c

@@ -3744,6 +3744,21 @@ on_power_save_mode_changed (MetaMonitorManager *monitor_manager,
     meta_kms_discard_pending_page_flips (kms);
 }
 
+void
+meta_renderer_native_reset_modes (MetaRendererNative *renderer_native)
+{
+  MetaRenderer *renderer = META_RENDERER (renderer_native);
+  MetaBackend *backend = meta_renderer_get_backend (renderer);
+  MetaBackendNative *backend_native = META_BACKEND_NATIVE (backend);
+  MetaKms *kms = meta_backend_native_get_kms (backend_native);
+  MetaKmsUpdate *kms_update;
+
+  kms_update = unset_disabled_crtcs (backend, kms);
+
+  if (kms_update)
+    post_pending_update (kms);
+}
+
 static MetaGpuKms *
 choose_primary_gpu_unchecked (MetaBackend        *backend,
                               MetaRendererNative *renderer_native)