gpu: ion: Take handle reference in ion_vma_open

When ion_vma_open is called, a reference to the handle in
the vma must be taken. Otherwise, if forking occurs,
ion_vma_close will be called twice which will leave one of
the calls with an invalid reference.

Change-Id: I9ab6e68ab6b1d2ac4bed4f45045b4b2ee39995e8
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>