iommu/amd: Use is_attach_deferred call-back

Implement call-back is_attach_deferred and use it to defer the
domain attach from iommu driver init to device driver init when
iommu is pre-enabled in kdump kernel.

Signed-off-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>