avcodec/cuvid: Fully re-initialize the parser after a flush.

I'm not really sure how this worked at all before, but we do need to
reinitalize the parser with the stream extradata.

Signed-off-by: Philip Langdale <philipl@overt.org>
Signed-off-by: Timo Rothenpieler <timo@rothenpieler.org>
