d3d11va: Use the proper decoding slice index

The decoding buffer index expected by D3D11VA is the one from the
ID3D11Texture2D not the one from the ID3D11VideoDecoderOutputView array
in AVD3D11VAContext.

Otherwise, when providing decoder slices that do not start from 0,
pictures appear in bogus order. For an invalid index crashes and
image corruption can occur.

Signed-off-by: Diego Biurrun <diego@biurrun.de>
