avcodec/ffv1enc: Fix storing RGB48 without explicitly set level

the bps value is only stored with level >= 1, using rgb48 with level 0 requires the
user app to keep track of the bps by external means, which does not always happen
also we force level >= 1 for other 16bps formats, so this is consistent.

Found-by: Jerome Martinez <jerome@mediaarea.net>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>