gpu: ion: Fix ion handle and buffer ref counting issue.

Handle and buffer are not ref counted correct during fork from
a process, which has mmap'ed ion memory.

Change-Id: Ida98f4639f29fef8397abd07bbf317c1baa0130e
Signed-off-by: Krishna Reddy <vdumpa@nvidia.com>
Reviewed-on: http://git-master/r/72643
Reviewed-by: Automatic_Commit_Validation_User
Reviewed-by: Scott Williams <scwilliams@nvidia.com>