[media] bttv: fix mutex use before init (BZ#24602)

Fix a regression where bttv driver causes oopses when loading, since it
were using some non-initialized mutexes. While it would be possible to
fix the issue, there are some other lock troubles, like to the presence of
lock code at free_btres_lock().

It is possible to fix, but the better is to just use the core-assisted
locking schema. This way, V4L2 core will serialize access to all
ioctl's/open/close/mmap/read/poll operations, avoiding to have two
processes accessing the hardware at the same time. Also, as there's just
one lock, instead of 3, there's no risk of dead locks.

The net result is a cleaner code, with just one lock.

Reported-by: Dan Carpenter <error27@gmail.com>
Reported-by: Brandon Philips<brandon@ifup.org>
Reported-by: Chris Clayton <chris2553@googlemail.com>
Reported-by: Torsten Kaiser <just.for.lkml@googlemail.com>
Tested-by: Chris Clayton <chris2553@googlemail.com>
Tested-by: Torsten Kaiser <just.for.lkml@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>