[Nouveau] stable? kms: take mode_config mutex in connector hotplug path

[poma]

https://github.com/skeggsb/nouveau/commit/9862b21
https://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git/commit/drivers/gpu/drm/nouveau/nouveau_connector.c?id=0a882cad

this was sent to @stable,
but test result is:

nouveau: Unknown symbol mutex_lock (err 0)
nouveau: Unknown symbol mutex_lock_interruptible (err 0)

4.4.3-201.fc22.x86_64+debug

Backport faux?