intel-iommu: Only avoid flushing device IOTLB for domain ID 0 in caching mode

In caching mode, domain ID 0 is reserved for non-present to present
mapping flush. Device IOTLB doesn't need to be flushed in this case.

Previously we were avoiding the flush for domain zero, even if the IOMMU 
wasn't in caching mode and domain zero wasn't special.

Signed-off-by: Yu Zhao <yu.zhao@intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>