Monitor: don't shortcut set_power_save if the value is the same as the old one

We don't get notifications from X11 when the mode is reset, so
our cached value can get stale. To work around that, always forward
requests to the backend (and let it deal with ignoring the change
if wanted)

https://bugzilla.gnome.org/show_bug.cgi?id=707649
Rename META_POWER_SAVE_UNKNOWN to UNSUPPORTED

It conveys more clearly what this value means.

https://bugzilla.gnome.org/show_bug.cgi?id=707649