drm/mediatek: mtk_dsi: Create connector for bridges

Use the drm_bridge_connector helper to create a connector for pipelines
that use drm_bridge. This allows splitting connector operations across
multiple bridges when necessary, instead of having the last bridge in
the chain creating the connector and handling all connector operations
internally.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>