drm/mediatek: no change parent rate in round_rate() for MT2701 hdmi phy

This is the third step to make MT2701 HDMI stable.
We should not change the rate of parent for hdmi phy when
doing round_rate for this clock. The parent clock of hdmi
phy must be the same as it. We change it when doing set_rate
only.

Signed-off-by: Wangyan Wang <wangyan.wang@mediatek.com>
Signed-off-by: CK Hu <ck.hu@mediatek.com>