drm/mediatek: dp: change mtk_dp_driver to static

mtk_dp_driver is only used in mtk_dp.c now, change it
to static.

Fixes: f70ac097 ("drm/mediatek: Add MT8195 Embedded DisplayPort driver")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Acked-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220913134929.1970187-1-yangyingliang@huawei.com