drm/msm: Use dma_sync_sg_for_device() to flush cache for new buffers

With the conversion of the master device to use the arm iommu
ops we lose the benefit of a cache flush using the dma_map_sg
trick. Plus we end up mapping all the GPU buffers into the master
DMA device which is admittedly sub optimal.

Change-Id: Ifa0e4aa229eb64a05daddd1d6d8168930c298150
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Alan Kwong <akwong@codeaurora.org>