xfree86/modes: Don't clobber gamma LUT of compatibility output's CRTC

If the driver calls xf86HandleColormaps, CMapChangeGamma updates the HW
gamma LUT of all CRTCs via xf86RandR12LoadPalette. However,
xf86RandR12ChangeGamma was then clobbering the gamma LUT of the RandR
1.2 compatibility output's CRTC with the gamma curves computed from the
screen's global gamma values.

Fix this by bailing if xf86RandR12LoadPalette is installed.

Fixes: 02ff0a5d "xf86RandR12: Fix XF86VidModeSetGamma triggering a
                     BadImplementation error"
(cherry picked from commit 30044b22)