drm/i915: Relocate CHV CGM gamma masks

CGM_PIPE_GAMMA_RED_MASK & co. are misplaced. Move then below the
relevant register. And while at it add the degamma counterparts.

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/20200925131656.10022-8-ville.syrjala@linux.intel.com