iommu/vt-d: Allocate si_domain in init_dmars()

This seperates the allocation of the si_domain from its
assignment to devices. It makes sure that the iommu=pt case
still works in the kdump kernel, when we have to defer the
assignment of devices to domains to device driver
initialization time.

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