drm/mediatek: enhance the HDMI driving current

In order to improve 4K resolution performance,
we have to enhance the HDMI driving current
when clock rate is greater than 165MHz.

Signed-off-by: Junzhi Zhao <junzhi.zhao@mediatek.com>
Signed-off-by: Bibby Hsieh <bibby.hsieh@mediatek.com>