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>