drm/i915: Prep for C8 palette readout

drm/i915: Prep for C8 palette readout

Add the approproate c8_planes checks to make the LUT
code ready for C8 palette readout. Note we currently
lack the actual c8_planes readout, so this won't work
yet. But no harm in making the code somewhat more ready
for the day when we do get c8_planes readout.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221114153732.11773-14-ville.syrjala@linux.intel.com
Reviewed-by: Uma Shankar <uma.shankar@intel.com>