dell-wmi: Add a WMI event code for display on/off

Some new Dell AIO systems have a button that generates a WMI event to
turn the LCD on/off.

Signed-off-by: Alex Hung <alex.hung@canonical.com>
Acked-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>