drm/omap: venc: Register a drm_bridge

In order to integrate with a chain of drm_bridge, the internal VENC
encoder has to expose the mode valid, fixup and set, the enable and
disable and the get modes operations through the drm_bridge API.
Register a bridge at initialisation time to do so.

Most of those operations are removed from the omap_dss_device as they
are now called through the drm_bridge API by the DRM atomic helpers. The
only exception is the .get_modes() operation that is still invoked
through the omap_dss_device-based pipeline.

For the time being make the next bridge in the chain optional as the
VENC output is still based on omap_dss_device. The create_connector
argument to the bridge attach function is also ignored for the same
reason. This will be changed later when removing the related
omapdrm-specific display drivers.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Tested-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200226112514.12455-38-laurent.pinchart@ideasonboard.com