kms/update: Make each MetaKmsUpdate update a single device

For now feedbacks from an update are combined, meaning we might lose
error information. The feedback API may have to be reconsidered and
redesigned when planes gets a more front seat position.

This means we need to avoid trying to post updates if we're in power
save mode, as it may be empty.

Note that this is an intermediate state during refactoring that aims to
introduce atomic mode setting support, and we'll stop combining
feedbacks completely in the future.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1488>

src/backends/native/meta-kms-device.c

@@ -55,6 +55,12 @@ struct _MetaKmsDevice
 
 G_DEFINE_TYPE (MetaKmsDevice, meta_kms_device, G_TYPE_OBJECT);
 
+MetaKms *
+meta_kms_device_get_kms (MetaKmsDevice *device)
+{
+  return device->kms;
+}
+
 MetaKmsImplDevice *
 meta_kms_device_get_impl_device (MetaKmsDevice *device)
 {