x86/amd-iommu: Add function to complete a tlb flush

This patch adds a function to the AMD IOMMU driver which
completes all queued commands an all IOMMUs a specific
domain has devices attached on. This is required in a later
patch when per-domain flushing is implemented.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>