gnome-rr: Remove unused GNOME_RR_DPMS_DISABLED

GNOME_RR_DPMS_DISABLED is one of the possible values of GnomeRRDpmsMode
that's not currently used, and would trigger an assertion if passed to
gnome_rr_screen_set_dpms_mode().

https://bugzilla.gnome.org/show_bug.cgi?id=751794