h264parse: Fix up bad colorimetry values

Some streams have weird colorimetry settings, specifying BT601 color matrix with BT709 transfer function, even though that is equivalent to vanilla BT601 colorimetry. For BT601 and BT709, match up equivalent transfer functions a bit, so that streams can output colorimetry=bt601 instead of weird colorimetry=2:4:5:4

https://devstreaming-cdn.apple.com/videos/streaming/examples/bipbop_16x9/bipbop_16x9_variant.m3u8 is an example stream, as are various GStreamer-created ones from before gst-plugins-base!724 (merged)

The particular problem this fixes is making the streams work with decoders like v4l2h264dec that only expose a few colorimetry values on the sink pad and will flat-ou t refuse streams with 'weird' colorimetry