gpu: ion: Only set the dma_address if it isn't already set

Certain heaps such as the carveout heap need to set the dma_address
of the scatterlist in a way without relying on sg_phys. Only
set the dma_address if it isn't already set.

Change-Id: I9d971557d08c79447908523f69a8f1cc5fa79f6a
CRs-Fixed: 469300
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>