drm/omap: Add support for drm_panel

Hook up drm_panel support in the omapdrm driver. The change is
relatively simply as the way has been paved by drm_bridge support
already. In addition to looking up, attaching to and detaching from the
panel, we only need to add panel support in the connector .get_modes()
handler, take connector bus flags (set by the panel) into account, and
enable/disable the panel in the encoder enable/disable operations
handlers.

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>