drm/i915: Skip the linear degamma LUT load on ICL+

Don't load the linear degamma LUT on ICL. The hardware no longer
has any silly linkages between the CSC enable and degamma LUT
enable so the degamma LUT is only needed when it's actually
enabled.

Also add comments to explain the situation on GLK.

v2: Drop useless parens around 1<<16
v3: Add missing const

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190327155045.28446-11-ville.syrjala@linux.intel.com