gpu: ion: Incrememnt user refcount during vma_open/vma_close

Ion allows multiple mmaps of the same buffers. This means that
incrementing the usermap count in mmap and decrementing it in
release is not sufficient as release will only ever be called
once. Fix this by only doing the reference count change in the
vma_open and close which are called whenever mmap is called.

Change-Id: I6d17c0f563d0b90481d8e092c1e206f2b33c8e00
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>