drm/i915/dsb: Use non-posted register writes for legacy LUT

drm/i915/dsb: Use non-posted register writes for legacy LUT

The DSB has problems writing the legacy LUT. The two workarounds
I've discoverted are:
- write each entry twice back to back
- use non-posted writes

Let's use non-posted writes as that seems a bit more standard.

TODO: measure which is faster

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230606191504.18099-14-ville.syrjala@linux.intel.com
Reviewed-by: Uma Shankar <uma.shankar@intel.com>