Looks like a userspace bug can trigger this somehow during a mode
switch, causing: EVO complaint -> semaphores get out of sync ->
entire display stalled.

We likely want to be even stricter than this (or at least deal
better if EVO rejects our request), but I'll save that for the
drm_plane/atomic conversion and just fix the bug that I already
know can be triggered.
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>