drm/xe: Replace list_del_init with list_del for userptr.invalidate_link cleanup

This list isn't used again, list_del is the proper call.

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