ui: disable 'lazyredraw' during ui_refresh.

Could also try `do_redraw = true` instead of save/restore `p_lz`, but
the nice thing about save/restore of `p_lz` is that it is "atomic".
The semantics of `do_redraw` are not clear to me.

Closes #4884
References #6202
References https://github.com/neovim/neovim/pull/6202#issuecomment-284379503
References #3929 #5692 #6157
References #5866

src/nvim/ui.c

@@ -193,7 +193,12 @@ void ui_refresh(void)
   }
 
   row = col = 0;
+
+  int save_p_lz = p_lz;
+  p_lz = false;  // convince redrawing() to return true ...
   screen_resize(width, height);
+  p_lz = save_p_lz;
+
   for (UIWidget i = 0; (int)i < UI_WIDGETS; i++) {
     ui_set_external(i, ext_widgets[i]);
   }