The various clut handling functions like a setup
consistent with the x-screen color depth. Otherwise
we observe improper sampling in the gamma tables
at depth 30.

Therefore replace hard-coded bitsPerRGB = 8 by actual
bits per channel scrn->rgbBits. Also use this for call
to xf86HandleColormaps().

Tested for uxa and sna at depths 8, 16, 24 and 30 on
IvyBridge, and tested at depth 24 and 30 that xgamma
and gamma table animations work, and with measurement
equipment to make sure identity gamma ramps actually
are identity mappings at the output.

v2: Also deal with X-Server 1.19 and earlier, which as of
    v1.19.6 lack a fix to color palette handling and can
    not deal with depths/bpc > 24/8 bpc. On < 1.20 we skip
    xf86HandleColormaps() setup at > 8 bpc. This disables
    color palette handling on such servers at > 8 bpc, but
    still keeps RandR gamma table handling intact.

    Tested on 1.19.6 and 1.20.0 to do the right thing.
Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>