monitor: Honor current power save mode

Ignore power mode changes when they match the current mode. While this is a good idea by itself to avoid pointless mode sets it also helps #900 (closed) as the overlapping events triggered by screen-saver-manager and monitor-manager (triggered by g-s-d) trip up the drm mode setting in the compositor.

We'll revisit the compositor side after catching up with recent wlroots (phoc!410 (merged)).

Signed-off-by: Guido Günther agx@sigxcpu.org