The src3 and step arguments to vector_fmul_add_add() are always zero
and one, respectively.  This removes these arguments from the function,
simplifies the code accordingly, and renames the function to better
match the new operation.

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@20061 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b