drm/i915: Reuse ilk_gamma_mode() on ivb+

Apart from the split gamma mode ivb+ LUTs work just like ilk+ LUTs.
So let's handle the special case, and then just fall back to
ilk_gamma_mode() to avoid having to duplicate the same logic.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221026113906.10551-9-ville.syrjala@linux.intel.com


Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>