[media] cx88: gracefully reject attempts to use unregistered cx88-blackbird driver

It should not be possible to enter mpeg_open and acquire core->lock
without the blackbird driver being registered, so just error out if it
is not.  This makes the code more readable and should prevent the bug
fixed by the patch "hold device lock during sub-driver initialization"
from resurfacing again.

Similarly, if we enter mpeg_release and acquire core->lock then either
the blackbird driver is registered (since open files keep it loaded)
or the sysadmin forced the driver's removal.  In the latter case the
state will be inconsistent and this is worth a loud warning.

Tested-by: Andi Huber <hobrom@gmx.at>
Tested-by: Marlon de Boer <marlon@hyves.nl>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>