gpu: ion: Add call back for unmapping user mappings

When mmap is called to remove a mapping, the mmap code
takes care of removing all the entries so it isn't
necessary to call unmap versions. However, there may be
cases where heaps need to do other buffer clean up when
a userspace buffer is unmapped. Add an unmap_user function
to the ion heap ops. This callback needs to happen in
vma_close, so move the decrementing of the overall user map
call back as well.

Change-Id: I8e5716774dd973828f76e03ec43e8e8ecf8c7936
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>