avcodec/ffv1enc: mark RGB48 support as non-experimental

Resulting bitstream was tested with a conformance checker
using the last draft of FFV1 specifications.

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

Also the files are already in the wild, and decoder support is
thus needed. And with decoders widely supporting it, there is no
advantage in not allowing it in the encoder.
The exact bitstream format may change in future versions of the
spec, if improvments are found.
