drm/mediatek: set dpi pin mode to gpio low to avoid leakage current

Config dpi pins mode to output and pull low when dpi is disabled.
Aovid leakage current from some dpi pins (Hsync Vsync DE ... ).

Signed-off-by: Jitao Shi <jitao.shi@mediatek.com>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>