iommu/amd: Add flush counters to struct dma_ops_domain

The counters are increased every time the TLB for a given
domain is flushed. We also store the current value of that
counter into newly added entries of the flush-queue, so that
we can tell whether this entry is already flushed.

Signed-off-by: Joerg Roedel <jroedel@suse.de>