arm64: dma-mapping: swap arguments to __get_dma_pgprot

The arguments to __get_dma_pgprot in arm64 are reversed from those in
the corresponding arm version. Match them up to facilitate the re-use of
some other code (like the IOMMU mapping code) from arm in arm64.

Change-Id: Idc8645c3455bda4e991f3e04eee5726b2b398092
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>