dma-mapping: direct calls for dma-iommu

Directly call into dma-iommu just like we have been doing for dma-direct
for a while.  This avoids the indirect call overhead for IOMMU ops and
removes the need to have DMA ops entirely for many common configurations.

Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>