drm/mediatek: Remove duplicated include

Remove mtk_drm_ddp.h which is included more than once

Fixes: 9aef5867 ("drm/mediatek: drop use of drmP.h")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wang Hai <wanghai38@huawei.com>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>