Show the physical device node of backlight class device.

Create symbol link from backlight class device to ACPI video device.

More and more laptops are shipped with multiple ACPI
video devices, while we export only one of them to userspace.

With this patch applied, we can know which ACPI video device
is used by "cat /sys/class/backlight/acpi_video0/device/path".

Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>