[Intel-gfx] Depth 30 colormap handling fixes for servers 1.20+ and < 1.20

[Mario Kleiner]

Hi,

finally here's an updated patch that for depth 30 now works on both
Server 1.20 with the full colormap + gamma table handling, and for
servers < 1.20 with the RandR gamma tables working fine and the colormap
processing skipped.

This one successfully tested on sna and uxa with both server 1.20.0 and
server 1.19.6.

I assume this one will be replaced by Ville's ddx+kmswork anyway soonish,
but until that is done, this one keeps things at least testable without
crashes and other problems. I use it with my own intel-kms 10 bit lut poc
hacks for measurements and to test that Mesa's depth 30 stuff doesn't
break.

Thanks,
-mario