[radeonhd] [PATCH v2] Force backlight control on some devices

OK, previous idea of enabling backlight control on *every* PANEL was really poor one. Egbert suggested we can use new flag for this in case we decide to keep problematic devices in quirks table. This adds pretty little code and works great for my Sony, even without AtomBIOS. So, are you fine with this? -- Rafał Miłecki