vc1dec: factorize picture pointer selection in vc1_mc_4mv_chroma()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
vc1dec: fix current ptr selection in vc1_mc_4mv_chroma()

No sample tried shows a difference

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>