gpu: ion: Delay unmapping from IOMMU.

Due to limitations in the multimedia architecure
clients might not know when a buffer can be unmapped
from the IOMMU. In addition, the multimedia architecture
causes unnecessary mappings/unmappings for the same buffers
which reduces framerates.

Add logic to delay unmapping from the IOMMU
until buffer memory is freed and unmap any outstanding
mappings to avoid virtual memory address space leak.

Change-Id: Idaeae269d9ba623e25a0cb087a89b4cbb63915af
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>