avformat/movenc: factor out find_fps()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
lavd: pulse audio encoder

Signed-off-by: Lukasz Marek <lukasz.m.luki@gmail.com>
Reviewed-by: Stefano Sabatini <stefasab@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>