drm/i915: Add gamma/degamma readout for bdw+

Read out the gamma/degamma LUT on bdw+. Now that the
{pre,post}_csc_lut match the hardware LUT size even
in split gamma mode this is trivial.

v2: deal with {pre,post}_csc_lut
    split gamma is no longer a problem
v3: add 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/20221114153732.11773-11-ville.syrjala@linux.intel.com