drm/i915: Add i9xx_lut_8()

We have a nice little helper to compute a single LUT entry
for everything except the 8bpc legacy gamma mode. Let's
complete the set.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191107151725.10507-5-ville.syrjala@linux.intel.com


Reviewed-by: Emil Velikov <emil.velikov@collabora.com>