drm/xe: Use Xe ordered workqueue for rebind worker

A mix of the system unbound wq and Xe ordered wq was used for the
rebind, only use the Xe ordered wq. This will ensure only 1 rebind is
occuring at a time providing a somewhat clunky work around for short
comings in TTM wrt to memory contention. Once the TTM memory contention
is resolved we should be able to use a dedicated non-ordered workqueue.

Also add helper to queue rebind worker to avoid using wrong workqueue
going forward.

Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>