drm/rockchip: Fix Kconfig dependencies for display-port encoders

The DP-helper module has been replaced by the display-helper module.
So the driver have to select it.

Reported-by: Hulk Robot <hulkci@huawei.com>
Fixes: 1e0f6642("drm/display: Introduce a DRM display-helper module")
Signed-off-by: Ren Zhijie <renzhijie2@huawei.com>
Reviewed-by: Andy Yan <andy.yan@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220507100910.93705-1-renzhijie2@huawei.com