avcodec/nvdec: Add support for decoding monochrome av1

avcodec/nvdec: Add support for decoding monochrome av1

The nvidia hardware explicitly supports decoding monochrome content,
presumably for the AVIF alpha channel. Supporting this requires an
adjustment in av1dec and explicit monochrome detection in nvdec.

I'm not sure why the monochrome path in av1dec did what it did - it
seems non-functional - YUV440P doesn't seem a logical pix_fmt for
monochrome and conditioning on chroma sub-sampling doesn't make sense.
So I changed it.

I've tested 8bit content, but I haven't found a way to create a 10bit
sample, so that path is untested for now.