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

[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"
(654a731be1a0b6f606f3f3d12b50db08f2ae3c3) 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 <[email protected]>
Signed-off-by: Kamil Debski <[email protected]>
Signed-off-by: Sylwester Nawrocki <[email protected]>
Signed-off-by: Mauro Carvalho Chehab <[email protected]>