avcodec/dnxhddec: Disable 12bit by default

The DC VLC table used is too small, fixing this requires a sample,
thus request a sample.
Some samples are said to work even though the table has the wrong size, thus
this is left enabled if the user enables experimental features.

Fixes: 2abd25478c62a675f335fac00b467023/asan_static-oob_10aff98_1227_8811480c6ef1e970a7977ceb7e5a9958.mxf

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Approved-by: kurosu
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
