drm: fix driver deadlock with AIGLX and reclaim_buffers_locked

Bugzilla Bug #9457

Add refcounting of user waiters to the DRM hardware lock, so that we can use
DRM_LOCK_CONT flag more conservatively.

Also add a kernel waiter refcount that if nonzero transfers the lock for the
kernel context when it is released. This is useful when waiting for idle and can be used for very simple fence object driver implementations for the new memory manager

Signed-off-by: Dave Airlie <airlied@linux.ie>