iommu/amd: Split init_iommu_group() from iommu_init_device()

For a PCI device, aliases from the IVRS table won't be populated
into dma_alias_devfn until after iommu_init_device() is called on
each device.  We therefore want to split init_iommu_group() to
be called from a separate loop immediately following.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Cc: stable@vger.kernel.org # 3.17
Signed-off-by: Joerg Roedel <jroedel@suse.de>