OK, so the NV4A actually runs into a different issue - the MPEG class can only
take linear memory, but the PCI GART is paged. So it can't deal the CMD/DATA
bo's. When moving those to VRAM (in mesa), the NV4A is fine. I plugged a NV42
in with much worse results. On boot I get:

https://hastebin.com/datuzivebu.sql

[   10.110345] nouveau 0000:04:00.0: mpeg: ch -1 [unknown] 03100023 ffffffff
00000001 ffffffff
[   10.191580] nouveau 0000:04:00.0: mpeg: MSRCH 0xffffffff

And just all kinds of fail. This is with some local patches to print the extra
stuff out. Feels like it's not getting powered on (all those 0xffffffff's) in
MC.