drm/i915: Split i9xx_read_lut_8() to gmch vs. ilk variants

To mirror the load_luts path let's clone an ilk+ version
from i9xx_read_lut_8(). I guess the extra branch isn't a huge
issue but feels better to make a clean split.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200303173313.28117-4-ville.syrjala@linux.intel.com
Reviewed-by: Swati Sharma <swati2.sharma@intel.com>