Since linux 4.14, the nouveau driver deadlocks at boot.

Downstream report:
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1763189?comments=all

The culprit is this commit:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=af2405af07d168e2905f2ac9494d3a804a549f0f

In short, drm_fb_helper_add_one_connector() and
drm_fb_helper_remove_one_connector() were incompatibly changed to add locking
of a mutex that previously the caller was expected to lock.  The i915 and
radeon drivers were updated to no longer lock the mutex, but nouveau was not
updated.