drm/mediatek: adjust ddp clock control flow

display hardware clock will not unprepare when
crtc is disable, until crtc is destroyed.
with this patch, hard clock will disable and unprepare
at the same time.

Signed-off-by: Yongqiang Niu <yongqiang.niu@mediatek.com>
Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
Signed-off-by: CK Hu <ck.hu@mediatek.com>