drm/mediatek: Modify dpi power on/off sequence.

Modify dpi power on/off sequence so that the first gpio operation will
take effect.

Fixes: 6bd4763f ("drm/mediatek: set dpi pin mode to gpio low to avoid leakage current")
Signed-off-by: Xinlei Lee <xinlei.lee@mediatek.com>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>