Dynamic SQL:
select 'set identity_insert ['+s.name+'].['+o.name+'] off'
from sys.objects o
inner join sys.schemas s on s.schema_id=o.schema_id
where o.[type]='U'
and exists(select 1 from sys.columns where object_id=o.object_id and is_identity=1)
Then copy & paste the resulting SQL into another query window and run