iommu/vt-d: Detect pre enabled translation

Add code to detect whether translation is already enabled in
the IOMMU. Save this state in a flags field added to
struct intel_iommu.

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