drm/msm/gem: Remove submit_unlock_unpin_bo()

The only point it is called is before pinning objects, so the "unpin"
part of the name is fiction.  Just remove it and call submit_cleanup_bo()
directly.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/568330/