gpu: ion: Add api to get client from file id.

Change-Id: Ifb3bc3d7fa54cddda7e3f0acd34dbcc4b1ed39d3
Signed-off-by: Krishna Reddy <vdumpa@nvidia.com>
Reviewed-on: http://git-master/r/72644
Reviewed-by: Automatic_Commit_Validation_User
Reviewed-by: Aleksandr Frid <afrid@nvidia.com>