drm/xe/tests: Fix migrate test

Pass a valid vm to xe_migrate_update_pgtables.

Resolves NPD crash seen with igt@xe_live_ktest@migrate

Reviewed-by: Brian Welty <brian.welty@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Pallavi Mishra <pallavi.mishra@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>