V4L/DVB (7122): saa7134-empress: Remove back lock

videobuf functions at close() method already locks videobuf. It makes no sense
to keep the locking at empress close() method.

There is also a lock at open() method. I'm not sure if it is safe to remove the
locking there.

Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>