drm/rockchip: remove rockchip_drm_encoder_get_mux_id

It is replaced by drm_of_encoder_active_endpoint_id.

Suggested-by: Daniel Kurtz <djkurtz@chromium.org>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Tested-by: Yakir Yang <ykk@rock-chips.com>
[for dw_hdmi-rockchip]
Acked-by: Mark Yao <mark.yao@rock-chips.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>