gpu: ion: remove IOMMU Heap

The IOMMU heap has been merged into the System heap since there was so
much overlap between the two. Remove the IOMMU heap. ION_IOMMU_HEAP_ID
will now alias to ION_SYSTEM_HEAP_ID.

Change-Id: I1cea1d97eba1488f8d5928e6a0f31a18b0fd92e5
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>