drm/dp_mst: Sanitize error return during payload addition

Return an error during payload addition if the payload port isn't
found. This shouldn't change the behavior since only the i915 driver
checks the return value, printing an error message in case of a failure.

While at it simplify the control flow.

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/20230913223218.540365-2-imre.deak@intel.com