drm/omap: Add support for drm_bridge

Hook up drm_bridge support in the omapdrm driver. Despite the recent
extensive preparation work, this is a rather intrusive change, as the
management of outputs needs to be adapted through the driver to handle
both omap_dss_device and drm_bridge.

Connector creation is skipped when using a drm_bridge, as the bridge
creates the connector internally. This creates issues with systems that
split connector operations (such as modes retrieval and hot-plug
detection) across different bridges. These systems can't be supported
using drm_bridge for now (their support through the omap_dss_device
infrastructure is not affected), this will be fixed in subsequent
changes.

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