Actually disable docstring prefix normalization with -S + fix instability (#3168)

The former was a regression I introduced a long time ago. To avoid
changing the stable style too much, the regression is only fixed if
--preview is enabled

Annoyingly enough, as we currently always enforce a second format pass if
changes were made, there's no good way to prove the existence of the
docstring quote normalization instability issue. For posterity, here's
one failing example:

    --- source
    +++ first pass
    @@ -1,7 +1,7 @@
     def some_function(self):
    -    ''''<text here>
    +    """ '<text here>

         <text here, since without another non-empty line black is stable>

    -    '''
    +    """
         pass
    --- first pass
    +++ second pass
    @@ -1,7 +1,7 @@
     def some_function(self):
    -    """ '<text here>
    +    """'<text here>

         <text here, since without another non-empty line black is stable>

         """
         pass

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>