Draft: nouveau: check for dead channels when waiting on fences
Should make the desktop more responsive if clients start to wait on dead channels. This might even solve some of the "frozen desktop" issues we see regularly.
Depends on this libdrm MR: drm!188
WIP, because it depends on a changed libdrm API and I need to bump the version reqs or check against a version when compiling or something...
- We actually don't stop gracefully and rely on the application to crash, which still can cause freezes until that happens.