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"