hdpvr: register the video node at the end of probe

commit 280847b532433ffe7a22795f926327805a127162 upstream.

Video nodes can be used at once after registration, so make sure the full
initialization is done before registering them.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
[bwh: Backported to 3.2: adjust filename, context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>