drm/i915: Always enable CGM CSC on CHV

On CHV toggling the CGM CSC on/off while the pipe is running leads
to underruns. Looks like we'd have to do the toggling strictly inside
the start_of_vblank-frame_start window to avoid this, but that window
is less than a scanline so there's no way we can guarantee hitting it.

As a workaround let's just leave the CGM CSC permanently enabled.
Fortunately the CGM gamma/degamma units don't seem to suffer from
this malady.

I also tried turning off CGM unit clock gating, but that did not
help.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230413164916.4221-5-ville.syrjala@linux.intel.com


Reviewed-by: Uma Shankar <uma.shankar@intel.com>