dxva: support DXGI_FORMAT_420_OPAQUE decoding

Some devices (some phones, apparently) will support only this opaque
format. Of course this won't work with CLI, because copying data
directly is not supported.

Automatic frame allocation (setting AVCodecContext.hw_device_ctx) does
not support this mode, even if it's the only supported mode. But since
opaque surfaces are generally less useful, that's probably ok.

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>