avcodec/fic: avoid 2 additions per idct row

before:
5225 decicycles in IDCT, 32756 runs, 12 skips

after:
5057 decicycles in IDCT, 32765 runs, 3 skips

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