UPSTREAM: 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


(cherry picked from commit 8bde4442)

BUG=b:332903004, b:322223603
TEST=Verify the link training fallback for UHBR<->non-UHBR rates

Change-Id: Ib4c5a0b6e15623e9f1c32803cb1d7aa2fa19dcec
Signed-off-by: Rakshith M O <rakshith.m.o@intel.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/5803706


Reviewed-by: Manasi Navare <navaremanasi@chromium.org>
Reviewed-by: Sean Paul <sean@poorly.run>
Signed-off-by: Hubert Mazur <hmazur@google.com>