OMAP: DSS2: HDMI: use default dividers

Use default regn and regm2 dividers in the hdmi driver if the board file
does not define them.

Cc: Mythri P K <mythripk@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>