iommu/vt-d: avoid dev iotlb logic for domains with no dev iotlbs

This patch avoids taking the device_domain_lock in iommu_flush_dev_iotlb()
for domains with no dev iotlb devices.

Signed-off-by: Omer Peleg <omer@cs.technion.ac.il>
[gvdl@google.com: fixed locking issues]
Signed-off-by: Godfrey van der Linden <gvdl@google.com>
[mad@cs.technion.ac.il: rebased and reworded the commit message]
Signed-off-by: Adam Morrison <mad@cs.technion.ac.il>
Reviewed-by: Shaohua Li <shli@fb.com>
Reviewed-by: Ben Serebrin <serebrin@google.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>