avcodec/nvdec: Explicitly mark codecs that support 444 output formats

avcodec/nvdec: Explicitly mark codecs that support 444 output formats

With the introduction of HEVC 444 support, we technically have two
codecs that can handle 444 - HEVC and MJPEG. In the case of MJPEG,
it can decode, but can only output one of the semi-planar formats.

That means we need additional logic to decide whether to use a
444 output format or not.