drm/i915/display/vlv: fix pixel overlap register update

To update properly bits in the register the mask should be used
to clear old value and then the result should be or-ed with new
value, for such updates there is separate helper intel_de_rmw.

Signed-off-by: Andrzej Hajda <andrzej.hajda@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221219092428.2515430-1-andrzej.hajda@intel.com