vc1dec: Partial rewrite of vc1_pred_mv()

The current code is a generalization of the earlier progressive MV
prediction code. This was supposed to predict MVs for both interlaced
and progressive pictures. But the interlaced MV prediction is buggy
and works mostly by luck.

This partially fixes interlaced MV prediction.

Signed-off-by: Anton Khirnov <anton@khirnov.net>