Add locking. The main lock is dev->mode_config.config_lock. It should be

Add locking.  The main lock is dev->mode_config.config_lock.  It should be
held across any operations that modify mode lists, crtc config, output
config, etc.  It should be taken at high level entry points (currently just
initial config and user IOCTL).

Seems to work ok on my system, but needs more testing (with lockdep) and
review from some fresh eyes.