dxva2: don't check for DXVA_PictureParameters->wDecodedPictureIndex

This structure is well defined by Microsoft at:
http://msdn.microsoft.com/en-us/library/windows/hardware/ff564012(v=vs.85).aspx
Thus, the wDecodedPictureIndex member is guaranteed to exist.

Also, both the MPEG-2 and VC-1 hwaccel decoders depend on this struct member,
but only the VC-1 decoder was disabled if the check failed.

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