iommu/vt-d: Enable Translation only if it was previously disabled

Do not touch the TE bit unless we know translation is
disabled.

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