drm/i915: Clean up 12.4bit precision palette defines

Use consistent bit definitions for the 12.4bit precision palette bits.
We just define these alongside the ilk/snb register definitions and
point to those from the icl+ superfine segment defines (and we also
already pointed to them from the ivb+ precision palette 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/20221114153732.11773-4-ville.syrjala@linux.intel.com