drm/i915: Refactor LUT read functions

Extract all the 'hw value -> LUT entry' stuff into small helpers
to make the main 'read out the entire LUT' loop less bogged down
by such mundane details.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200303173313.28117-8-ville.syrjala@linux.intel.com


Reviewed-by: Swati Sharma <swati2.sharma@intel.com>