drm/mediatek: add helpers for coverting from the generic components

define helpers for converting from 'mtk_ddp_comp' to 'mtk_disp_ovl'
define helpers for converting from 'mtk_ddp_comp' to 'mtk_disp_rdma'

Signed-off-by: YT Shen <yt.shen@mediatek.com>
Acked-by: CK Hu <ck.hu@mediatek.com>