drm/i915: Add glk+ degamma readout

Read out the degamma LUT on glk+. No state cheker as of yet since
it requires dealing with the glk csc vs. degamma mess.

v2: deal with post_csc_lut
v3: add icl_has_{pre,post}_csc_lut(*) helpers

Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221114153732.11773-9-ville.syrjala@linux.intel.com