interpreter: Allow FileMode to be passed to _get_kwarg_install_mode

This will happen as we transition from doing the conversion in the
function body to using the KwargInfo to make that change. If we get one
just return it.

mesonbuild/interpreter/interpreter.py

@@ -1898,6 +1898,8 @@ This will become a hard error in the future.''' % kwargs['input'], location=self
     def _get_kwarg_install_mode(self, kwargs: T.Dict[str, T.Any]) -> T.Optional[FileMode]:
         if kwargs.get('install_mode', None) is None:
             return None
+        if isinstance(kwargs['install_mode'], FileMode):
+            return kwargs['install_mode']
         install_mode: T.List[str] = []
         mode = mesonlib.typeslistify(kwargs.get('install_mode', []), (str, int))
         for m in mode: