acer-wmi: Blacklist backlight on Acer Aspire 1520 & 1360 series

A newer BIOS for these laptops adds ACPI-WMI support to them. However, it does
not add support for the backlight via the EC, and we have no way to detect
this on older machines, so blacklist it from them.

Signed-off-by: Carlos Corbacho <carlos@strangeworlds.co.uk>
Signed-off-by: Len Brown <len.brown@intel.com>
Signed-off-by: Andi Kleen <ak@linux.intel.com>