drm/xe: Fix DSB buffer coherency

Add the scanout flag to force WC caching, and add the memory barrier
where needed.

Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240913114754.7956-2-maarten.lankhorst@linux.intel.com


Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>