drm/i915: Shuffle chv_cgm_gamma_pack() around a bit

Move chv_cgm_gamma_pack() next to the other CGM gamma functions.
Right now it's stuck in the middle of the CGM degamma functions.

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/20200925131656.10022-7-ville.syrjala@linux.intel.com