gpu: ion: Map a range into the IOMMU

Instead of mapping 1 4K page at a time into the IOMMU create a
scatterlist and map everything at once. This will be more efficient.

Change-Id: I8e83066869dd6f7a479bad22a66e4c70cc5973b5
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>