drm - Fix a second X lockup w/radeon ttm

drm - Fix a second X lockup w/radeon ttm

* Code which releases bo->reserved and wakes up waiters was
  not interlocked against wait_event_common() used to wait
  for the release.  This can result in a race where the
  release occurs inbetween the wait's test and its sleep,
  preventing the wait from ever waking up.

* This is different from the recent dev_pager_mtx deadlock which
  was recently fixed, but in the same code path.

* Roll the release of bo->reserved into a static function
  and throw the bo->event_queue.lock around the actual
  release to fix the race.