iommu/amd: Move interrupt setup code into seperate function

For interrupt remapping the enablement of the IOMMU MSI
interrupt needs to be deferred because the IOMMU itself will
be initialized before the io-apics are up and running. So
the code to setup the MSI is moved seperated from the
hardware-setup routine now.

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