OMAP: DSS2: HDMI: Use dss_device clock configuration for HDMI PLL parameters

Move some of the configurable HDMI PLL parameters to dssdev.clock struct.
Cleanup the function hdmi_compute_pll() by using the parameters defined in the
board file and do some cosmetic modifications.

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