drm/i915/dsb: Stop with the RMW

We don't want to keep random bits set in DSB_CTRL. Stop the
harmful RMW.

Also flip the reverse & around to appease my ocd.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221216003810.13338-2-ville.syrjala@linux.intel.com


Reviewed-by: Animesh Manna <animesh.manna@intel.com>