Get iommu from g_iommus for deferred flush

deferred_flush[] uses the iommu seq_id to index, so its iommu is fixed and can get it from g_iommus.

Signed-off-by: Weidong Han <weidong.han@intel.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>