iommu/dma: Squash __iommu_dma_{map,unmap}_page helpers

The remaining internal callsites don't care about having prototypes
compatible with the relevant dma_map_ops callbacks, so the extra
level of indirection just wastes space and complictaes things.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Joerg Roedel <jroedel@suse.de>