drm/mediatek: Ensure bridge disable happends before suspend

Make sure bridge_disable will be called before suspend by calling
drm_mode_config_helper_suspend() in .prepare callback.

Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://patchwork.kernel.org/project/linux-mediatek/patch/20220629190545.478113-1-hsinyi@chromium.org/


Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>