OMAPDSS: pass pck to dss fck clock calc

We need the required pixel clock rate when calculating the dss fclk on
SoCs that have a dedicated DSS PLL.

This patch changes the code to pass the pck to the calc functions. The
pck rate is taken into use in the next patch.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>