drm/msm/dp: use correct data clock rate

In case the maximum data clock rate has been set by debug module,
use the minimum of what the target supports and debug clock rate.
This is needed to support different clock rate requirements.

Change-Id: I396c73a6807460ed64c420a088d956ed635ade3e
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>