iommu/amd: Use iommu_group_get_for_dev()

The common iommu_group_get_for_dev() allows us to greatly simplify
our group lookup for a new device.  Also, since we insert IVRS
aliases into the PCI DMA alias quirks, we should alway come up with
the same results as the existing code.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Cc: Joerg Roedel <joro@8bytes.org>
Signed-off-by: Joerg Roedel <jroedel@suse.de>