Fixup sysfs output registration

Put off registering new outputs with sysfs until they're properly configured,
or we may get duplicates if the type hasn't been set yet (as is the case with
SDVO initialization).  This also means moving de-registration into the cleanup
function instead of output destroy, since the latter occurs during the normal
course of setup when an output isn't found (and therefore not registered with
sysfs yet.