[media] v4l: vsp1: Register pipe with output WPF

The DRM object does not register the pipe with the WPF object. This is
used internally throughout the driver as a means of accessing the pipe.
As such this breaks operations which require access to the pipe from WPF
interrupts.

Register the pipe inside the WPF object after it has been declared as
the output.

Fixes: ff7e97c9 ("[media] v4l: vsp1: Store pipeline pointer in rwpf")

Signed-off-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>