drm/mediatek: remove flag CLK_SET_RATE_PARENT for MT2701 hdmi phy

This is the first step to make MT2701 hdmi stable.
The parent rate of hdmi phy had set by DPI driver.
We should not set or change the parent rate of MT2701 hdmi phy,
as a result we should remove the flags of "CLK_SET_RATE_PARENT"
from the clock of MT2701 hdmi phy.

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