drm: Destroy the correct mutex name in drm_dp_mst_topology_mgr_destroy

It looks like one of the topology manager mutexes may have been renamed
during a rebase, but the destruction function wasn't updated with the
new name:

   error: ‘struct drm_dp_mst_topology_mgr’ has no member named
   ‘delayed_destroy_lock’

Fixes: 50094b5d

 ("drm/dp_mst: Destroy topology_mgr mutexes")
Cc: Lyude Paul <lyude@redhat.com>
Cc: Sean Paul <sean@poorly.run>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190925224617.24027-1-matthew.d.roper@intel.com