iommu/amd: Remove the leftover of bypass support

The AMD iommu dma_ops are only attached on a per-device basis when an
actual translation is needed.  Remove the leftover bypass support which
in parts was already broken (e.g. it always returns 0 from ->map_sg).

Use the opportunity to remove a few local variables and move assignments
into the declaration line where they were previously separated by the
bypass check.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Joerg Roedel <jroedel@suse.de>