drm/i915: Extract ilk_lut_10()

Extract a helper to calculate the ILK+ 10bit gamma LUT entry.
It's already duplicated twice, and soon we'll have more.

v2: s/it/bit/ (Matt)

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190401200231.2333-2-ville.syrjala@linux.intel.com


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