drm/nouveau: add per-channel mutex, use to lock access to drm's channel

drm/nouveau: add per-channel mutex, use to lock access to drm's channel

This fixes a race condition between fbcon acceleration and TTM buffer
moves.  To reproduce:

- start X
- switch to vt and "while (true); do dmesg; done"
- switch to another vt and "sleep 2 && cat /path/to/debugfs/dri/0/evict_vram"
- switch back to vt running dmesg

We don't make use of this on any other channel yet, they're currently
protected by drm_global_mutex.  This will change in the near future.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>