iommu/amd: No need to wait iommu completion if no dte irq entry change

This is a clean up. In get_irq_table() only if DTE entry is changed
iommu_completion_wait() need be called. Otherwise no need to do it.

Signed-off-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>