drm/i915: Clean up legacy palette defines

Use consistent bit definitions for the legacy gamma LUT. We just
define these alongside the pre-ilk register definitions and point
to those from the ilk+ defines.

Also use the these appropriately in the LUT entry pack/unpack
functions.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221114153732.11773-2-ville.syrjala@linux.intel.com