nouveau: rewrite nouveau_stateobj to use BEGIN_RING properly

nouveau: rewrite nouveau_stateobj to use BEGIN_RING properly

- The previous solution was hacky and didn't do subchannel autobinding.
- The beheaviour should match what libdrm_nouveau does closely.
- The solution remains statically sized, but when debugging is on it will check
for abuse.

Signed-off-by: Maarten Maathuis <madman2003@gmail.com>