drm/bridge: panel: export drm_panel_bridge_connector

The function was unexported and was causing link failures for pl111 (and
probably the other user tve200) in a module build.

Fixes: d383fb5f

 ("drm: get drm_bridge_panel connector via helper")
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Linux Walleij <linux.walleij@linaro.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mihail Atanassov <mihail.atanassov@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191210144834.27491-1-mihail.atanassov@arm.com