media: rcar-vin: Register media device when all sub-devices bound

The media device is not usable by userspace before all devices involved
in capture are present in the system. Move registering of the media
device to the async complete callback.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>