drm/mediatek: Move mtk_ddp_comp_init() from sub driver to DRM driver

Some ddp component exist in both display path and other path, so
sub driver should not directly call DRM driver's function. Moving
mtk_ddp_comp_init() from sub driver to DRM driver to achieve this.

Signed-off-by: CK Hu <ck.hu@mediatek.com>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>