iommu/vt-d: Use dma_direct for bypass devices

The intel-iommu driver currently has a partial reimplementation
of the direct mapping code for devices that use pass through
mode.  Replace that code with calls to the relevant dma_direct
routines at the highest level.  This means we have exactly the
same behvior as the dma direct code itself, and can prepare for
eventually only attaching the intel_iommu ops to devices that
actually need dynamic iommu mappings.

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