drm/i915: Polish CHV .load_luts() a bit

drm/i915: Polish CHV .load_luts() a bit

It irks me to use crtc_state_is_legacy_gamma() inside the guts
of the CHV color management code. Let's get rid of it and instead
just consult cgm_mode to figure out if we want to enable the pipe
gamma or the CGM gamma.

Also CHV display engine is based on i965/g4x so we should fall back
to the i965 path when the CGM gamma is not used.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191107151725.10507-3-ville.syrjala@linux.intel.com
Reviewed-by: Swati Sharma <swati2.sharma@intel.com>