iommu/amd: Split out PCI related parts of IOMMU initialization

For interrupt remapping the relevant IOMMU initialization
needs to run earlier at boot when the PCI subsystem is not
yet initialized. To support that this patch splits the parts
of IOMMU initialization which need PCI accesses out of the
initial setup path so that this can be done later.

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