patch 8.0.1002: unnecessarily updating screen after timer callback

Problem:    Unnecessarily updating screen after timer callback.
Solution:   Check if calling the timer sets must_redraw.

src/terminal.c

@@ -669,7 +669,7 @@ write_to_term(buf_T *buffer, char_u *msg, channel_T *channel)
 	    update_cursor(term, TRUE);
 	}
 	else
-	    redraw_after_callback();
+	    redraw_after_callback(TRUE);
     }
 }