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.

BugLink: http://bugs.launchpad.net/bugs/1702541Signed-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>
(backported from commit ab9bb11c)
Signed-off-by: Alex Hung <alex.hung@canonical.com>
Acked-by: Colin Ian King <colin.king@canonical.com>
Acked-by: AceLan Kao <acelan.kao@canonical.com>
Acked-by: Po-Hsu Lin <po-hsu.lin@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>

drivers/platform/x86/dell-wmi.c

@@ -90,6 +90,9 @@ static const struct key_entry dell_wmi_legacy_keymap[] __initconst = {
 	{ KE_IGNORE, 0xe025, { KEY_RESERVED } },
 	{ KE_IGNORE, 0xe026, { KEY_RESERVED } },
 
+	/* LCD Display On/Off Control key */
+	{ KE_KEY,    0xe027, { KEY_DISPLAYTOGGLE } },
+
 	{ KE_IGNORE, 0xe02e, { KEY_VOLUMEDOWN } },
 	{ KE_IGNORE, 0xe030, { KEY_VOLUMEUP } },
 	{ KE_IGNORE, 0xe033, { KEY_KBDILLUMUP } },