drm/dp_mst: Fix memory leak in drm_dp_mst_topology_mgr_destroy()

drm/dp_mst: Fix memory leak in drm_dp_mst_topology_mgr_destroy()

We need to call drm_dp_mst_topology_mgr_set_mst(mgr, false) when
destroying the topology manager in order to ensure that the root mstb
and all of it's descendents are actually destroyed, and additionally to
try to make sure that we leave the hub in a clean state.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Daniel Vetter <daniel@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20181211235026.21758-1-lyude@redhat.com