drm/omap: Move HPD disconnection handling to omap_connector

On HDMI outputs, CEC support requires notification of HPD signal
deassertion. The HPD signal can be handled by various omap_dss_device
instances in the pipeline, and all of them forward HPD events to the
OMAP4 internal HDMI encoder.

Knowledge of the DSS internals need to be removed from the
omap_dss_device instances in order to migrate to drm_bridge. To do so,
move HPD handling for CEC to the omap_connector.

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>