gpu: ion: Map everything into IOMMU with 64K pages.

Due to performance issues with 4K pages and the IOMMU we should use
64K pages. However, minimum allocation from ION is 4K so instead of
mapping on demand we need to map the full heap into the IOMMU when
the first request for mapping comes. Only unmap everything from the
IOMMU when the last buffer is freed.

CRs-fixed: 348606
Change-Id: Ic1793f5caaff2f69bf1fb7e8c8b3bd03378131b8
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>