OMAPDSS: HDMI: rewrite HDMI PLL calculation code

The code calculating HDMI PLL parameters has always been very confusing.
Now that we are implementing a common PLL library for the DSS, it's
important that the PLL code is understandable.

This patch rewrites the calculation code, and removes a few hacks that
were used there.

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