arm64: dts: rockchip: enable DP for rk3399-gru

Enable cdn_dp and create a cdn-dp-sound for the DP audio. Delete the
endpoints between dp and vopL for gru, since we want the DP only use
VOP big, which can support 4K mode.

Signed-off-by: Chris Zhong <zyw@rock-chips.com>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
[dropped vop-hacks]
Signed-off-by: Heiko Stuebner <heiko@sntech.de>