This register needs to be updated with masked writes.

This was found by code inspection and comparison with Bspec and
doesn't seem to fix any known issue.
Signed-off-by: NMika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: NMichel Thierry <michel.thierry@intel.com>
[danvet: Add note about impact.]
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>