OMAPDSS: DISPC: pass pclk to calc_core_clk()

In order to make the scaling calculations independent of the current
hardware configuration (e.g. which manager is connected to this output),
we need to change the calc funcs to get all the variables needed for the
calculations via parameters.

This patch changes calc_core_clk() function to get pclk as a parameter.

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