avcodec/utils: Display pixel format information as none ... if unknown instead of omitting it

This is similar to 20a5956b8daeee4cb59d6fa00ec809b02c04d7f8

The difference this change makes affects only rather few files
but looks like:
Stream #0:1: Video: mjpeg, 90k tbr, 90k tbn, 90k tbc
vs.
Stream #0:1: Video: mjpeg, none(bt470bg/unknown/unknown), 90k tbr, 90k tbn, 90k tbc

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>