drm/selftests/test-drm_dp_mst_helper: Fix memory leak in sideband_msg_req_encode_decode

Avoid leaking the "out" variable if it is not possible to allocate
the "txmsg" variable.

Fixes: 09234b88 ("drm/selftests/test-drm_dp_mst_helper: Move 'sideband_msg_req_encode_decode' onto the heap")
Addresses-Coverity-ID: 1475685 ("Resource leak")
Signed-off-by: José Expósito <jose.exposito89@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20220108165812.46797-1-jose.exposito89@gmail.com