gpu: ion: Add map_dma support in IOMMU heap

IOMMU is missing support for ion_map_dma.
ION clients need support for this api.

Add functions to implement ion_map_dma and
ion_unmap_dma in the IOMMU heap.

Change-Id: If5723f0d7ec1145e9c9d8b88a42b57ab0f2c2da7
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>