x86/amd-iommu: Use 2-level page tables for dma_ops domains

The driver now supports a dynamic number of levels for IO
page tables. This allows to reduce the number of levels for
dma_ops domains by one because a dma_ops domain has usually
an address space size between 128MB and 4G.

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