gpu: ion: use module_platform_driver to simplify the code

module_platform_driver() makes the code simpler by eliminating
boilerplate code.

Change-Id: I55ad02bdc05dd35de8e451f952b7e8363df656c9
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Git-commit: 0a7dde3d881d173c9300bf1d46017cb593d3e52b
Git-repo: http://android.googlesource.com/kernel/common/


Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>