drm/xe/selftests: hold rpm for ccs_test_migrate()

The GPU job will keep the device awake, however assumption here is that
caller of xe_migrate_clear() is also holding mem_access.ref otherwise we
hit the asserts in xe_sa_bo_flush_write() prior to the job construction.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>