iommu/vt-d: Remove unnecassary qi clflushes

According to the manual: "Hardware access to ...  invalidation queue ...
are always coherent."

Remove unnecassary clflushes accordingly.

Signed-off-by: Nadav Amit <namit@vmware.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>