GnomeRR: force the DPMS mode change to mutter always

Our cached DPMS mode value can be wrong in face of changes by
other clients we haven't seen yet, so always send the change
to mutter, and let it optimize away if needed (in practice,
it should never optimize, because the only reliable source
of DPMS state is the kernel driver, and several API calls
can reset DPMS without any change notification)

https://bugzilla.gnome.org/show_bug.cgi?id=726055