[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 <[e-mail]>
Signed-off-by: Kamil Debski <[e-mail]>
Signed-off-by: Sylwester Nawrocki <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

drivers/media/platform/s5p-mfc/s5p_mfc.c

···
 	q->type = V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE;
 	q->io_modes = VB2_MMAP;
 	q->drv_priv = &ctx->fh;
+	q->lock = &dev->mfc_mutex;
 	if (vdev == dev->vfd_dec) {
 		q->io_modes = VB2_MMAP;
 		q->ops = get_dec_queue_ops();