arm: dma-mapping: use proper api to get prot attribute

Use proper API to get the IOMMU protection attributes which are used in
pagetable mappings.

Change-Id: I755fd6ee030ed3d1a3f443ac3624512e0ba66ee4
Fixes: Ibf67f29a60b ("arm: dma-mapping: map sg lists into the SMMU as
virtually contiguous")
Signed-off-by: Charan Teja Reddy <charante@codeaurora.org>