gpu: ion: Add missing iommu heap destroy call

When a heap is destroyed a heap type specific
function is called to release memory that the heap
structures used. A case statement handling destroying
the iommu heap is missing. Add missing case statement.

Change-Id: I1aa54209a55db06becc570f65ecf0f740d2e9545
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>