arm64: dma-mapping: optimize unmap coherency check

Change arm_iommu_unmap_page to check the DMA attributes for buffer
coherency support instead of checking the iommu page table.

This change now requires that clients correctly set the
DMA_ATTR_FORCE_COHERENT or DMA_ATTR_FORCE_NON_COHERENT DMA
attributes in their unmap call if they used it with their map call.

This will improve the performance of the arm_iommu_unmap_page
call.

Change-Id: Ie96ef0a1906a8732b7b1eef7bed8aa85abee95f4
Signed-off-by: Liam Mark <lmark@codeaurora.org>