drivers/gpu/drm/via/via_video.c: fix off by one issue

"fx->lock" is used as the index in "dev_priv->decoder_queue[fx->lock]"
which is an array of "VIA_NR_XVMC_LOCKS" elements.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>