Add and use a mutex to protect the cx88-mpeg device list.  Previously
the BKL prevented races.

Based on work by Ben Hutchings <ben@decadent.org.uk>.
Tested-by: NAndi Huber <hobrom@gmx.at>
Tested-by: NMarlon de Boer <marlon@hyves.nl>
Signed-off-by: NJonathan Nieder <jrnieder@gmail.com>
Signed-off-by: NMauro Carvalho Chehab <mchehab@redhat.com>