iommu/vt-d: Loose requirement for flush queue initializaton

Currently if flush queue initialization fails, we return error
or enforce the system-wide strict mode. These are unnecessary
because we always check the existence of a flush queue before
queuing any iova's for lazy flushing. Printing a informational
message is enough.

Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>