Microsoft Surface Book has a tablet mode button. Print another message
once on this event instead of repeating "Unknown event...".

Unfortunately, proper support involves the _DSM method, which is not a
discoverable interface. Just print a warning for now.
Signed-off-by: NAndy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: NDarren Hart <dvhart@linux.intel.com>