drm: bridge: simple-bridge: Make connector creation optional

Make the connector creation optional to enable usage of the
simple-bridge with the DRM bridge connector helper.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200526011505.31884-8-laurent.pinchart+renesas@ideasonboard.com