drm/dp_mst: Fix down request message timeout handling

If receiving a reply for an MST down request message times out, the
thread receiving the reply in drm_dp_mst_handle_down_rep() could try to
dereference the drm_dp_sideband_msg_tx txmsg request message after the
thread waiting for the reply - calling drm_dp_mst_wait_tx_reply() - has
timed out and freed txmsg, hence leading to a use-after-free in
drm_dp_mst_handle_down_rep().

Prevent the above by holding the drm_dp_mst_topology_mgr::qlock in
drm_dp_mst_handle_down_rep() for the whole duration txmsg is looked up
from the request list and dereferenced.

v2: Fix unlocking mgr->qlock after verify_rx_request_type() fails.

Cc: Lyude Paul <lyude@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241203174632.2941402-1-imre.deak@intel.com