Backport PR #9409: Return value from format_display_data

I guess nothing was using this ;-)

IPython/core/formatters.py

@@ -966,7 +966,7 b' def format_display_data(obj, include=None, exclude=None):'
     """
     from IPython.core.interactiveshell import InteractiveShell
 
-    InteractiveShell.instance().display_formatter.format(
+    return InteractiveShell.instance().display_formatter.format(
         obj,
         include,
         exclude