iommu/vt-d: Use iommu_group_get_for_dev()

The IOMMU code now provides a common interface for finding or
creating an IOMMU group for a device on PCI buses.  Make use of it
and remove piles of code.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Joerg Roedel <jroedel@suse.de>