drm/mediatek: mtk_dsi: Use the drm_panel_bridge API

Replace the manual panel handling code by a drm_panel_bridge. This
simplifies the driver and allows all components in the display pipeline
to be treated as bridges, paving the way to generic connector handling.

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