vaapi_encode: Destroy output buffer pool before VA context

The buffers are created associated with the context, so they should be
destroyed before the context is.  This is enforced by the iHD driver.

libavcodec/vaapi_encode.c

@@ -1556,6 +1556,8 @@ av_cold int ff_vaapi_encode_close(AVCodecContext *avctx)
         vaapi_encode_free(avctx, pic);
     }
 
+    av_buffer_pool_uninit(&ctx->output_buffer_pool);
+
     if (ctx->va_context != VA_INVALID_ID) {
         vaDestroyContext(ctx->hwctx->display, ctx->va_context);
         ctx->va_context = VA_INVALID_ID;
@@ -1566,8 +1568,6 @@ av_cold int ff_vaapi_encode_close(AVCodecContext *avctx)
         ctx->va_config = VA_INVALID_ID;
     }
 
-    av_buffer_pool_uninit(&ctx->output_buffer_pool);
-
     av_freep(&ctx->codec_sequence_params);
     av_freep(&ctx->codec_picture_params);