ACPI / video: Add a quirk to force acpi-video backlight on Dell XPS L421X

Just like the Dell XPS 15 (L521X) the Dell XPS 14 (L421X) needs to use
the acpi-video backlight interface rather then the native one for backlight
control to work, add a quirk for this.

Link: https://bugzilla.redhat.com/show_bug.cgi?id=1272633
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/video_detect.c

@@ -232,6 +232,15 @@ static const struct dmi_system_id video_detect_dmi_table[] = {
 			  "900X3C/900X3D/900X3E/900X4C/900X4D"),
 		},
 	},
+	{
+	 /* https://bugzilla.redhat.com/show_bug.cgi?id=1272633 */
+	 .callback = video_detect_force_video,
+	 .ident = "Dell XPS14 L421X",
+	 .matches = {
+		DMI_MATCH(DMI_SYS_VENDOR, "Dell Inc."),
+		DMI_MATCH(DMI_PRODUCT_NAME, "XPS L421X"),
+		},
+	},
 	{
 	 /* https://bugzilla.redhat.com/show_bug.cgi?id=1163574 */
 	 .callback = video_detect_force_video,