arm64: dma-mapping: Fix cache maintenance issue

Ensure that sg_dma_address() is called after iommu_dma_map_sg() so that it
gets a valid dma address. __iommu_sync_sg_for_device uses this to determine
whether cache maintenance is required for a buffer.

Change-Id: I2f770d2560e35bc53470ccca7f0cd78c7a2ee0a1
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>