xfree86/modes: Make colormap/gamma glue code work with RandR disabled

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: 62f44052 ("xfree86/modes: Move gamma initialization to
                      xf86RandR12Init12 v2")
Tested-by: Mariusz Bialonczyk <manio@skyboo.net>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 41dafcc2)