drm/omap: move HDMI PLL calc function to pll.c

Move hdmi_pll_compute(), used to calculate the config for HDMI PLL, from
hdmi_pll.c to pll.c, with the name of dss_pll_calc_b(), to make it
available to non-HDMI users.

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