drm/i915: Make ilk_load_luts() deal with degamma

Make ilk_load_luts() ready for a degamma lut. Currently we never
have one, but soon we may get one from readout, and I think we
may want to change the state computation such that we may end up
with one even when userspace has simply supplied a gamma lut.

At least the code now follows the path laid out by the ivb/bdw
counterpars.

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/20221024161514.5340-2-ville.syrjala@linux.intel.com