vaapi_encode: Add support for max QP in rate control

This was added in libva 2.1.0 (VAAPI 1.1.0).  Use AVCodecContext.qmax,
matching the existing behaviour for qmin, and clean up the defaults so
that we only pass min/max when explicitly set.

libavcodec/vaapi_encode.c

@@ -1436,6 +1436,9 @@ static av_cold int vaapi_encode_init_rate_control(AVCodecContext *avctx)
         .initial_qp        = 0,
         .min_qp            = (avctx->qmin > 0 ? avctx->qmin : 0),
         .basic_unit_size   = 0,
+#if VA_CHECK_VERSION(1, 1, 0)
+        .max_qp            = (avctx->qmax > 0 ? avctx->qmax : 0),
+#endif
     };
     vaapi_encode_add_global_param(avctx, &ctx->rc_params.misc,
                                   sizeof(ctx->rc_params));