drm: rcar-du: Use generic drm_connector_register_all() helper

Now that a generic drm_connector_register_all() helper exists we may safely
substitute it for the driver-specific implementation of connectors plugging
in sysfs.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: David Airlie <airlied@linux.ie>
Cc: linux-renesas-soc@vger.kernel.org
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1461068693-11260-4-git-send-email-abrodkin@synopsys.com