drm/i915: Clean up various indexed LUT registers

Use REG_BIT() & co. for the LUT index registers, and also
use the REG_FIELD_PREP() stuff a bit more consistently when
generating the values for said registers.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221123152638.20622-5-ville.syrjala@linux.intel.com


Reviewed-by: Uma Shankar <uma.shankar@intel.com>