OMAP: DSS2: HDMI: change regn definition

regn divider is currently programmed to the registers without change,
but when calculating clock frequencies it is used as regn+1.

To make this similar to how DSI handles the dividers this patch changes
the regn value to be used as such for calculations, but the value
programmed to registers is regn-1.

This simplifies the clock frequency calculations, makes it similar to
DSI, and also allows us to use regn value 0 as undefined.

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

include/video/omapdss.h

@@ -520,6 +520,7 @@ struct omap_dss_device {
 		} dsi;
 
 		struct {
+			/* regn is one greater than TRM's REGN value */
 			u16 regn;
 			u16 regm2;
 		} hdmi;