arm64: consolidate __swiotlb_mmap

Since commit 9d3bfbb4

 ("arm64: Combine coherent and non-coherent
swiotlb dma_ops"), __dma_common_mmap is no longer shared between two
callers, so roll it into the remaining one.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>