gpu: ion: Add support for IOMMU in CP heap

ION clients allocating from a content protection
heap (cp heap) must be able to map the memory to
an IOMMU when IOMMU are being used.

Add callbacks to CP heap to support IOMMU mapping.

Change-Id: I5dd70096edcab4866a436ae4447f9a6e00e2f0ec
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>