protocol: wl_subcompositor.get_subsurface gives a role

Reword the conditions to make use of the definition of "role".

It is still forbidden to create more than one wl_subsurface for a
wl_surface at a time.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>