OMAPDSS: DISPC: add new clock calculation code

Add new way to iterate over DISPC clock divisors. dispc_div_calc()
provides a generic way to go over all the divisors, within given pixel
clock range. dispc_div_calc() will call a callback function for each
divider set, making the function reusable for all use cases.

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