drm/xe/tests: Use proper batch base address

In xe_migrate_sanity_kunit test, use proper batch base address
by considering usm case.

Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>