drm/dp_mst: Skip CSN if topology probing is not done yet

[Why]
During resume, observe that we receive CSN event before we start topology
probing. Handling CSN at this moment based on uncertain topology is
unnecessary.

[How]
Add checking condition in drm_dp_mst_handle_up_req() to skip handling CSN
if the topology is yet to be probed.

Cc: Lyude Paul <lyude@redhat.com>
Cc: Harry Wentland <hwentlan@amd.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: stable@vger.kernel.org
Signed-off-by: Wayne Lin <Wayne.Lin@amd.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240626084825.878565-3-Wayne.Lin@amd.com