The bridge needs both its pads connected to be able to stream data.
Enforcing this is useful to produce an error when no sensor is
connected.

Fixes: af54b4f4 ("media: sunxi: Add support for the A31 MIPI CSI-2 controller")
Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>