[media] s5p-mfc: Fix NULL pointer dereference caused by not set q->lock

The patch "media: s5p-mfc: use vb2_ops_wait_prepare/finish helper"
(654a731b

) introduced a kernel panic.
The q->lock was set for just one queue, the other was not set thus causing
a NULL pointer dereference.

Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>