libavcodec/amfenc_hevc: Recommend values for min and max video quantizer scale.

Current settings makes bitrate larger than expected, more information:
https://github.com/HandBrake/HandBrake/issues/3447#issue-820490736

Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/amfenc_hevc.c

@@ -298,6 +298,8 @@ static const AVCodecDefault defaults[] = {
     { "b",          "2M"  },
     { "g",          "250" },
     { "slices",     "1"   },
+    { "qmin",       "-1"  },
+    { "qmax",       "-1"  },
     { NULL                },
 };
 static const AVClass hevc_amf_class = {