avcodec/cuvid: Check for non 420 chroma formats - they aren't supported

Despite the video parser seeming to correctly handle 422 and 444
chroma formats, the video decoder fails miserably to actually
decode frames - even though no errors are ever returned; you just
get frames showing unintialized garbage.

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