drm/i915/dsb: Add support for non-posted DSB registers writes

Writing specific transcoder registers (and as it turns out, the
legacy LUT as well) via DSB needs a magic sequence to emit
non-posted register writes. Add a helper for this.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230606191504.18099-11-ville.syrjala@linux.intel.com


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