drm/mediatek: Enable OVL_LAYER_SMI_ID_EN for multi-layer usecase

Enable OVL_LAYER_SMI_ID_EN for multi-layer usecase, without this patch,
ovl will hang up when more than 1 layer enabled.

Signed-off-by: Yongqiang Niu <yongqiang.niu@mediatek.com>
Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>