drm/ttm: Don't deadlock on recursive multi-bo reservations

Add an aid for the driver to detect deadlocks on multi-bo reservations
Update documentation.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Jerome Glisse <j.glisse@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>