drm/i915: Make ilk_read_luts() capable of degamma readout

Just like ivb+, ilk/snb can select whether the hw lut acts as
gamma or degamma. Make the readout cognizant of that fact.

v2: deal with pre_csc_lut
v3: use ilk_has_post_csc_lut() helper

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/20221114153732.11773-13-ville.syrjala@linux.intel.com