drm/rockchip: vop2: Fix the port mux of VP2

The port mux of VP2 should be RK3568_OVL_PORT_SET__PORT2_MUX.

Fixes: 604be855 ("drm/rockchip: Add VOP2 driver")
Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20240422101905.32703-2-andyshrk@163.com