drm/mediatek: Check plane visibility in atomic_update

Disable the plane if it's not visible. Otherwise mtk_ovl_layer_config()
would proceed with invalid plane and we may see vblank timeout.

Fixes: 119f5173

 ("drm/mediatek: Add DRM Driver for Mediatek SoC MT8173.")
Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>