drm/i915: Read out CHV CGM degamma

Since CHV has the dedicate CGM degamma unit readout is trivial.
Just do it.

v2: deal with post_csc_lut

Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221114153732.11773-10-ville.syrjala@linux.intel.com