drm: rcar-du: Don't fail probe in case of partial encoder init error

If an encoder fails to initialize the device can still be used without
the failed encoder. Don't propagate the error out of the probe function
but just skip the failed encoder.

As a special case a deferred probe request from the encoder is still
propagated out of the probe function in order not to break the deferred
probing mechanism.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>