gpu: ion: Export ion_client_create

Will enable modules to allocate memory with ion.

Signed-off-by: Johan Mossberg <johan.mossberg@stericsson.com>
[jstultz: modified patch to apply to staging directory]
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>