x86/amd-iommu: Fix passthrough mode

The data structure changes to use dev->archdata.iommu field
broke the iommu=pt mode because in this case the
dev->archdata.iommu was left uninitialized. This moves the
inititalization of the devices into the main init function
and fixes the problem.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>