drm/msm/gem: Cleanup submit_cleanup_bo()

Now that it only handles unlock duty, drop the superfluous arg and
rename it.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/568333/