drm/i915: s/chv_read_cgm_lut/chv_read_cgm_gamma/

chv_read_cgm_lut() specifically reads the CGM _gamma_ LUT so
let's rename it to reflect that fact. This also mirrors
the other direction's chv_load_cgm_gamma().

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200303173313.28117-6-ville.syrjala@linux.intel.com
Reviewed-by: Swati Sharma <swati2.sharma@intel.com>