drm/mediatek: update some variable name from ovl to comp

This patch update some variable name from ovl to comp

Because RDMA would be first HW in ddp, the naming ovl
should be change to comp.

Signed-off-by: Stu Hsieh <stu.hsieh@mediatek.com>
Signed-off-by: CK Hu <ck.hu@mediatek.com>