msm: mdss: fix overflow when reading pixel clock

Fix pixel clock value overflow when reading from the
dtsi. This fixes the frame rate calculations for 120 fps
panels; wrong frame rates were leading to low bandwidth
and mdp clock calculations, increasing the transfer
times and reducing the fps.

Change-Id: Ia41aae433f89e8970318224562b00cf0cb56767b
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>