monitor-manager: Don't notify unchanged power save mode

Merged Georges Basile Stavracas Neto requested to merge gbsneto/annoying-stuttering into master

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.

!674 (merged)

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