drm/i915: Clean up chv CGM (de)gamma defines

Add the missing ldw vs. udw information to the CGM (de)gamma
bit definitions to make it a bit easier to see which should
be used where.

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/20221114153732.11773-5-ville.syrjala@linux.intel.com