gpu: ion: Lower IOMMU heap priority

Video requires the IOMMU heap to be at a lower
priority so that video can try to allocate from
its MM carveout heap first and then if video needs
more memory and the MM heap is exhausted it will
automatically fall back on IOMMU heap.

Change the priority of the IOMMU heap so video
can fall back on IOMMU heap.

Change-Id: Id1fc19e2c3af18ced028132d1a1bdb6a3935380f
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>