drm/mediatek: mtk_dsi: Use simple encoder

The mtk_dsi driver uses an empty implementation for its encoder. Replace
the code with the generic simple encoder.

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>