acer-wmi: add 3 laptops to video backlight vendor mode quirk table
Acer Extensa 5235, TravelMate 5760 and Aspire 5750 laptop have broken _BCM
implemenation, the AML code wrote value to EC register but firmware didn't
change brighenss.
Fortunately, the brightness control works on those machines with vendor mode.
So, add this machine to video backlight vendor mode quirk table.