According to ffmpeg/libavcodec/mpeg12enc.c:155 (rev:2fd06be07311)
profile and level must be inited at the same time, so, -1 will be
returned for mp2v with width > 720 or height > 576. But, if we leave
this fields blank (i.e. FF_PROFILE_UNKNOWN and FF_LEVEL_UNKNOWN) avcodec
will init them correctly.

This partially reverts commit b1b72b63.
Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>