vp9: enable multi-thread decoding when refreshctx is equal to 0

In vp9_decode_frame function, ff_thread_finish_setup is not called
when refreshctx is equal to 0, and the next decoding thread can not
start work until the cunrrent frame has been decoded completely. So
ff_thread_finish_setup needs to be called to enable Multi-thread
decoding in this condition.
Signed-off-by: Di Wu <di1028.wu@samsung.com>
Signed-off-by: Reynaldo H. Verdejo Pinochet <reynaldo@osg.samsung.com>

libavcodec/vp9.c

@@ -3826,6 +3826,8 @@ static int vp9_decode_frame(AVCodecContext *ctx, void *frame,
         }
         s->prob_ctx[s->framectxid].p = s->prob.p;
         ff_thread_finish_setup(ctx);
+    } else if (!s->refreshctx) {
+        ff_thread_finish_setup(ctx);
     }
 
     do {