dma-iommu: remove __iommu_dma_mmap

The function has a single caller, so open code it there and take
advantage of the precalculated page count variable.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20201209112019.2625029-1-hch@lst.de


Signed-off-by: Will Deacon <will@kernel.org>