drm/omap: remove ovl_set_channel_out

At the moment we have ovl_set_channel_out() to configure the output
channel of an overlay. It makes sense to have this configuration as part
of the rest of overlay configuration, and in DSS6+ we need the output
channel when doing the other overlay configuration.

This patch adds a 'channel' parameter to ovl_setup(), so that all
overlay configuration is done via the same function, and removes the
ovl_set_channel_out().

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>