gpu: ion: allocate huge pages in iommu heap

As a performance optimization, use higher-order page allocations if
possible. This can reduce TLB misses and the total number of page
structures.

CRs-Fixed: 449035
Change-Id: I6b76ec69599a100fd7209161c4286284ae347be0
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>