utvideoenc: use ff_generate_len()

19% faster
smaller files
this may also fix possible integer overflows due to previous 32bit useage

Tested with libutvideo and our utvideo decoder, this patch does not change
decoder output in the test

Reviewed-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
