iommu/vt-d: Unwrap __get_valid_domain_for_dev()

We do find_domain() in __get_valid_domain_for_dev(), while we do the
same thing in get_valid_domain_for_dev().  No need to do it twice.

Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>