drm/dp_mst: Tune down error message during payload addition

If a sink is removed in the middle of payload addition the corresponding
AUX transfer will fail as expected, so tune the error message down to a
debug messge.

Cc: Wayne Lin <Wayne.Lin@amd.com>
Cc: Lyude Paul <lyude@redhat.com>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230913223218.540365-3-imre.deak@intel.com