drm/rockchip: cdn-dp: Move mutex_init to probe

We're trying to lock mutex when cdn-dp shutdown, so we need to make
sure the mutex is inited in cdn-dp's probe.

Signed-off-by: Jeffy Chen <jeffy.chen@rock-chips.com>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Reviewed-by: Chris Zhong <zyw@rock-chips.com>
Signed-off-by: Chris Zhong <zyw@rock-chips.com>