Patch to cherry-pick from master into 1.19 branch

[Mario Kleiner]

Hi Adam,

b5f9fcd50a999a00128c0cc3f6e7d1f66182c9d5 ("xfree86/modes: Adapt
xf86Randr12CrtcComputeGamma() for depth 30. (v2)")

would be really good to have in server 1.19.7, so xf86-video-intel/nouveau
can operate at screen depth 30 and keep gamma table updates via
xf86vidmode and randr functional, instead of the server switching to
identity gamma ramps for the whole depth 30 session and other weirdness.

Thanks,
-mario