Avoid a deadlock where DQBUF is holding the vb_lock while waiting on a QBUF
which also needs the vb_lock.  Reported by Hans Verkuil <hverkuil@xs4all.nl>.
Signed-off-by: Brandon Philips <bphilips@suse.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>