drm/dp_mst: Sanitize mgr->qlock locking in drm_dp_mst_wait_tx_reply()

Make the locking look symmetric with the unlocking.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200603211040.8190-2-imre.deak@intel.com