monitor-manager: Don't notify unchanged power save mode
Since 4cae9b5b, and indirectly before that as well, the MetaMonitorManager::power-save-mode-changed is emitted even when the power save mode didn't actually change.
On Wayland, this causes a mode set and therefore a stuttering. It became more proeminent with the transactional KMS code.
Only emit 'power-save-mode-changed' when the power save mode actually changed.