[media] cec: improve locking

- The global lock was used in cec_get_device when it should have
  used the devnode lock.
- cec_put_device also took the global lock, but since the release
  function takes that lock as well this could lead to a deadlock.
  Just don't take the lock here since there is no reason for it.
- cec_devnode_register() should take the global lock when clearing
  the bit in the global bitmap.
- In cec_devnode_unregister() place the devnode->(un)register tests
  and assignments under the devnode lock as well: this has to be
  in a critical block.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>