drm/mediatek: make implementation of recalc_rate() for MT2701 hdmi phy

Recalculate the rate of this clock, by querying hardware to
make implementation of recalc_rate() to match the definition.

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