Even though the name provided by either xdg-output or wl_output are
guaranteed to be unique, that might not be the case with output names
between different protocols, such as the one offered for DRM lease.

To avoid running into name conflicts, check that no other existing
output of the same name exists prior to changing the output name.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Part-of: <!1492>