drm/rockchip: cdn-dp: Don't try to release firmware when not loaded

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/1491481885-13775-6-git-send-email-jeffy.chen@rock-chips.com