media: sun6i-mipi-csi2: Require both pads to be connected for streaming

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>