Still use ACPI backlight control if _DOS doesn't exist

This is a regression introduced by commit
ea9f885

https://bugzilla.kernel.org/show_bug.cgi?id=43168

Some platforms don't have _DOS control method, but the ACPI
backlight still works.
We do not want to lose the backlight control ability on these platforms.

CC : Igor Murzov <intergalactic.anonymous@gmail.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>

drivers/acpi/video.c

@@ -567,8 +567,14 @@ acpi_video_bus_DOS(struct acpi_video_bus *video, int bios_flag, int lcd_flag)
 	video->dos_setting = arg0.integer.value;
 	status = acpi_evaluate_object(video->device->handle, "_DOS",
 		&args, NULL);
-	if (ACPI_FAILURE(status))
-		return -EIO;
+	if (ACPI_FAILURE(status)) {
+		/*
+		 * some platforms don't have _DOS, but the ACPI
+		 * backlight control still works
+		 */
+		ACPI_DEBUG_PRINT((ACPI_DB_INFO, "No _DOS\n"));
+		return 0;
+	}
 
 	return 0;
 }