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_h265.c

@@ -1148,6 +1148,8 @@ static const AVCodecDefault vaapi_encode_h265_defaults[] = {
     { "i_qoffset",      "0"   },
     { "b_qfactor",      "6/5" },
     { "b_qoffset",      "0"   },
+    { "qmin",           "-1"  },
+    { "qmax",           "-1"  },
     { NULL },
 };