vaapi_encode: Sync to input surface rather than output

While outwardly bizarre, this change makes the behaviour consistent
with other VAAPI encoders which sync to the encode /input/ picture in
order to wait for /output/ from the encoder.  It is not harmful on
i965 (because synchronisation already happens in vaRenderPicture(),
so it has no effect there), and it allows the encoder to work on
mesa/gallium which assumes this behaviour.

(cherry picked from commit 086e4b58b5)

libavcodec/vaapi_encode.c

@@ -109,10 +109,10 @@ static int vaapi_encode_wait(AVCodecContext *avctx,
     }
 
     av_log(avctx, AV_LOG_DEBUG, "Sync to pic %"PRId64"/%"PRId64" "
-           "(recon surface %#x).\n", pic->display_order,
-           pic->encode_order, pic->recon_surface);
+           "(input surface %#x).\n", pic->display_order,
+           pic->encode_order, pic->input_surface);
 
-    vas = vaSyncSurface(ctx->hwctx->display, pic->recon_surface);
+    vas = vaSyncSurface(ctx->hwctx->display, pic->input_surface);
     if (vas != VA_STATUS_SUCCESS) {
         av_log(avctx, AV_LOG_ERROR, "Failed to sync to picture completion: "
                "%d (%s).\n", vas, vaErrorStr(vas));