iommu/vt-d: Stop dmar_insert_dev_info() freeing domains on losing race

By moving this into get_domain_for_dev() we can make dmar_insert_dev_info()
suitable for use with "special" domains such as the si_domain, which
currently use domain_add_dev_info().

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>