drm/xe/migrate: fix MI_ARB_ON_OFF usage

Spec says: "This is a privileged command; it will not be effective (will
be converted to a no-op) if executed from within a non-privileged batch
buffer." However here it looks like we are just emitting it inside some
bb which was jumped to via the ppGTT, which should be considered
a non-privileged address space.

It looks like we just need some way of preventing things like the
emit_pte() and later copy/clear being preempted in-between so rather
just emit directly in the ring for migration jobs.

Bspec: 45716
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>