split out ff_hwaccel_pixfmt_list_420[] over individual codecs.

Not all hwaccels implement all codecs, so using one single list for
multiple such codecs means some codecs will be represented in the list,
even though they don't actually handle that codec. Copying specific
lists in each codec fixes that.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>