x86/amd-iommu: Fix IOMMU-API initialization for iommu=pt

This patch moves the initialization of the iommu-api out of
the dma-ops initialization code. This ensures that the
iommu-api is initialized even with iommu=pt.

Cc: stable@kernel.org
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>