dxva2: skip re-init when only aspect ratio changes

This improves DXVA-2 native playback with the WM ASF Reader,
as it would otherwise cause a severe delay on start of playback.
Always reduce the aspect ratio fraction to avoid reconnects

1920:1080 is the same as 16:9, but it would reconnect to change it.
Instead, always reduce to 16:9 immediately, and avoid a reconnect later.