drm/i915: Polish CHV CGM CSC loading

drm/i915: Polish CHV CGM CSC loading

Only load the CGM CSC based on the cgm_mode bit like we
do with the gamma/degamma LUTs. And make the function
naming and arguments consistent as well.

TODO: the code to convert the coefficients look totally
bogus. IIRC CHV uses two's complement format but the code
certainly doesn't generate that, so probably negative
coefficients are totally busted.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200303173313.28117-2-ville.syrjala@linux.intel.com
Reviewed-by: Swati Sharma <swati2.sharma@intel.com>