drm/i915: Add gamma/degamma readout for ivb/hsw

We now have all the code necessary for gamma/degamma readout on
ivb/hsw. Plug it all in. As with bdw+ the cooked {pre,post}_csc_lut
make this trivial even in split gamma mode.

Note that on HSW if IPS is enabled the hardware will hang if
you try to access the LUT in split gamma mode. Thus we need to
reorder the LUT readout vs. IPS enable steps.

v2: deal with {pre,post}_csc_lut
    split gamma is no longer a problem
    handle HSW IPS w/a
v3: use ilk_has_post_csc_lut() helper

Reviewed-by: Uma Shankar <uma.shankar@intel.com> #v1
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221114153732.11773-12-ville.syrjala@linux.intel.com