drm/omap: add dispc_channel_connected field to omap_dss_device

We want to remove the 'struct omap_overlay_manager' from
omap_dss_device. At the moment that field is used, among some other
uses, to see if the omap_dss_device is connected to an overlay manager.

To make it possible to remove the 'struct omap_overlay_manager' field,
this patch adds 'bool dispc_channel_connected' field to track the
connected-or-not status.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>