drm/i915: Make the CHV CGM CSC register writes lockless

drm/i915: Make the CHV CGM CSC register writes lockless

The CHV CGM CSC registers are single buffered and so we
may have to write them from the vblank worker, which
imposes very tight dealines. Drop the pointless locking
for the register accessess to reduce the overhead.
All the other registers we bash from the vblank worker
(LUTs) were already made lockless earlier.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220202111616.1579-3-ville.syrjala@linux.intel.com
Reviewed-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>