drm/rockchip: vop: Unprepare clocks when unbinding

The clocks are prepared when binding vop.

Signed-off-by: Jeffy Chen <jeffy.chen@rock-chips.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1491481885-13775-9-git-send-email-jeffy.chen@rock-chips.com