As vendor document indicate, when REF_CLK bit set 0, then DP
phy's REF_CLK should switch to 24M source clock.

But due to IC PHY layout mistaken, some chips need to flip this
bit(like RK3288), and unfortunately they didn't indicate in the
DP version register. That's why we have to make this little hack.
Signed-off-by: Yakir Yang <ykk@rock-chips.com>
Reviewed-by: Tomasz Figa <tomasz.figa@chromium.com>
Tested-by: Javier Martinez Canillas <javier@osg.samsung.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>