avcodec/d3d11va: use the proper slice index

The slice index expected by D3D11VA is the one from the texture not from the
array or texture/slices.

In VLC the slices we provide the decoder don't start from 0 and thus pictures
appear in bogus order. With possible crashes and corruptions when using an
invalid index.

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
