gpu: ion: system_heap: add support for 2MB allocations

Enable 2MB (order 9) allocations from the system heap. This improves the
performance of some heavyweight use cases. The benefit is especially
large for use cases where the buffer is mapped into an IOMMU since we
can leverage the 2MB IOMMU page table mappings.

Change-Id: I869733089b9ccba8090bbca80aac7b5f248b335e
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>