iommu/vt-d: Don't do early domain assignment if kdump kernel

When we copied over context tables from an old kernel, we
need to defer assignment of devices to domains until the
device driver takes over. So skip this part of
initialization when we copied over translation tables from
the old kernel.

Tested-by: ZhenHua Li <zhen-hual@hp.com>
Tested-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>