drm/omap: generalize dss_pll_calc_b()

dss_pll_calc_b() takes HDMI TMDS clock rate as a parameter. To make
dss_pll_calc_b() usable for non-HDMI users, change the function to take
clkout rate as parameter, and also change the current users of
dss_pll_calc_b() to accommodate that.

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