drm/xe: Use write-combine mapping when populating DPT

drm/xe: Use write-combine mapping when populating DPT

The fallback case for DPT backing store is a buffer object in system
memory buffer, which by default use a write-back CPU caching policy.

If this fallback gets triggered, and since there is currently no flushing,
the DPT writes made when pinning a buffer to display are not guaranteed to
be seen by the display engine.

To fix this, since both the local memory and the stolen memory DPT
placements already use write-combine, let us make the system memory option
follow suit by passing down the appropriate flag.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patch.msgid.link/20260324084018.20353-3-tvrtko.ursulin@igalia.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>