To prepare for making the connector creation optional, move the related
code out of adv7511_bridge_attach() to a separate function.

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/20200526011505.31884-3-laurent.pinchart+renesas@ideasonboard.com