drm/dp_mst: Handle torn-down topology gracefully in drm_dp_mst_topology_queue_probe()

drm/dp_mst: Handle torn-down topology gracefully in drm_dp_mst_topology_queue_probe()

A hotplug or link-loss event can tear down the MST topology
(setting mgr->mst_state = false and mgr->mst_primary = NULL) concurrently
with a caller invoking drm_dp_mst_topology_queue_probe(). Since the check
is already performed under mgr->lock, the condition is not a programming
error but a valid race -- the topology was valid when the caller decided
to call this function, but was torn down before the lock was acquired.

Replace the drm_WARN_ON() with a graceful early return. This eliminates
spurious kernel warnings and the resulting compositor crashes observed
when connecting/disconnecting DP MST monitors, while keeping the correct
behavior of doing nothing when MST is not active. A drm_dbg_mst() trace
is added so the skipped probe remains observable under MST debug logging.

The existing WARN_ON(mgr->mst_primary) in drm_dp_mst_topology_mgr_set_mst()
already catches the case where the topology is initialized twice, so no
diagnostic coverage is lost.

Fixes: dbaeef363ea5 ("drm/dp_mst: Add a helper to queue a topology probe")
Cc: Imre Deak <imre.deak@intel.com>
Cc: Lyude Paul <lyude@redhat.com>
Cc: stable@vger.kernel.org
Cc: intel-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Jonas Emilsson <jonas.emilsson@gmail.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/all/20260503034533.1023686-1-jonas.emilsson@gmail.com
Acked-by: Imre Deak <imre.deak@intel.com>
Link: https://patch.msgid.link/20260622140532.526722-1-luciano.coelho@intel.com
Signed-off-by: Maarten Lankhorst <dev@lankhorst.se>