iommu/vt-d: Make root entry visible for hardware right after allocation

In case there was an old root entry, make our new one
visible immediately after it was allocated.

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