soc / drm: mediatek: Move routing control to mmsys device

Provide a mtk_mmsys_ddp_connect() and mtk_mmsys_disconnect() functions to
replace mtk_ddp_add_comp_to_path() and mtk_ddp_remove_comp_from_path().
Those functions will allow DRM driver and others to control the data
path routing.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Acked-by: CK Hu <ck.hu@mediatek.com>
Tested-by: Anders Roxell <anders.roxell@linaro.org>
Reviewed-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>