ACPI video: Still use ACPI backlight control if _DOS doesn't exist

This fixes a regression in 3.4-rc1 caused by commit
ea9f885
(ACPI video: Harden video bus adding.)

Some platforms don't have _DOS control method, but the ACPI
backlight still works.
We should not invoke _DOS for these platforms.

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

Cc: Igor Murzov <[email protected]>
Cc: [email protected]
Signed-off-by: Zhang Rui <[email protected]>
Signed-off-by: Len Brown <[email protected]>

Author: zhang-rui

drivers/acpi/video.c

@@ -558,6 +558,8 @@ acpi_video_bus_DOS(struct acpi_video_bus *video, int bios_flag, int lcd_flag)
 	union acpi_object arg0 = { ACPI_TYPE_INTEGER };
 	struct acpi_object_list args = { 1, &arg0 };
 
+	if (!video->cap._DOS)
+		return 0;
 
 	if (bios_flag < 0 || bios_flag > 3 || lcd_flag < 0 || lcd_flag > 1)
 		return -EINVAL;