drm: rcar-du: Rename and document dpll_ch field

Document and re-name the 'dpll_ch' field to a more precise 'dpll_mask' for
consistency with the 'channels_mask' field defined in 'struct
rcar_du_device_info'.

Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>