libvpxdec: set CODEC_CAP_DR1 for vp9

it shares vp8_decode() with vp8 which already has the flag set
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/libvpxdec.c

@@ -144,6 +144,6 @@ AVCodec ff_libvpx_vp9_decoder = {
     .init           = vp9_init,
     .close          = vp8_free,
     .decode         = vp8_decode,
-    .capabilities   = CODEC_CAP_AUTO_THREADS | CODEC_CAP_EXPERIMENTAL,
+    .capabilities   = CODEC_CAP_AUTO_THREADS | CODEC_CAP_DR1 | CODEC_CAP_EXPERIMENTAL,
 };
 #endif /* CONFIG_LIBVPX_VP9_DECODER */