drm/tegra: dsi - Do not needlessly recompute pclk

In some cases the pixel clock used to not be correct, which is why it
had to be recomputed. It turns out that the reason why it wasn't correct
is that it was used wrongly. If used correctly there's not need for the
recomputation.

Signed-off-by: Thierry Reding <treding@nvidia.com>