msm: mdss: dp: fix div by zero error

If for some reasons like ioctl fuss test or reading dpcd caps failed,
max lane count supported might be zero. In such cases handle link clk
calculation gracefully.

Change-Id: I7cb08abce76025930681f6532c03708793d3acc4
Signed-off-by: Narender Ankam <nankam@codeaurora.org>