E.g. because Xinerama is enabled.
Fixes crash on startup and wrong colours in that case.
Bugzilla: https://bugs.freedesktop.org/100293
Bugzilla: https://bugs.freedesktop.org/100294
Fixes: 62f4405257 ("xfree86/modes: Move gamma initialization to
xf86RandR12Init12 v2")
Tested-by: Mariusz Bialonczyk <manio@skyboo.net>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>