drm/omap: Set dispc_channel_connect from DSS output connect handlers

The omap_dss_device.dispc_channel_connect field is used by DSS outputs
to fail the .enable() operation if they're not connected. Set the field
directly from the (dis)connect handlers of the DSS outputs instead of
going through the CRTC dss_mgr operations.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>