asus-wmi: Disable acpi-video backlight on desktop machines

Some Asus motherboards for desktop PC-s export an acpi-video interface
advertising backlight support. Test the dmi chassis-type and tell acpi-video
to not register a backlight interface on desktops.

https://bugzilla.redhat.com/show_bug.cgi?id=1097436

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>

drivers/platform/x86/asus-wmi.c

@@ -46,6 +46,7 @@
 #include <linux/platform_device.h>
 #include <linux/thermal.h>
 #include <linux/acpi.h>
+#include <linux/dmi.h>
 #include <acpi/video.h>
 
 #include "asus-wmi.h"
@@ -1734,6 +1735,7 @@ static int asus_wmi_add(struct platform_device *pdev)
 	struct platform_driver *pdrv = to_platform_driver(pdev->dev.driver);
 	struct asus_wmi_driver *wdrv = to_asus_wmi_driver(pdrv);
 	struct asus_wmi *asus;
+	const char *chassis_type;
 	acpi_status status;
 	int err;
 	u32 result;
@@ -1770,6 +1772,11 @@ static int asus_wmi_add(struct platform_device *pdev)
 	if (err)
 		goto fail_rfkill;
 
+	/* Some Asus desktop boards export an acpi-video backlight interface,
+	   stop this from showing up */
+	chassis_type = dmi_get_system_info(DMI_CHASSIS_TYPE);
+	if (chassis_type && !strcmp(chassis_type, "3"))
+		acpi_video_dmi_promote_vendor();
 	if (asus->driver->quirks->wmi_backlight_power)
 		acpi_video_dmi_promote_vendor();
 	if (!acpi_video_backlight_support()) {