I'm having the same problem with a clean install of Windows 8.1 and the latest Catalyst drivers.
HP Envy 15 model 1150nr laptop
Mobility Radeon 5830 graphics
Here's what is happening for me. Windows initially installs the " Microsoft basic Display Adapter" driver. With this driver, the brightness controls work fine, whether it's being changed from the kepboard function keys, from the Power Options screen, or from the Windows 8.1 settings menu (Win + I).
As soon as the driver is updated to the lastest Catalyst version, brightness controls stop working completely. I've tried every proposed solution found online to no avail.
Also tried using the latest beta version of Catalyst. Still doesn't work. I have updated all necessary drivers, including Intel chipset motherboard drivers.
When the latest Catalyst drivers are installed, brigtness controls just don't work. Reverting back to the MS basic display adapter fixes the issue immediately (without rebooting even), but is not an optimal driver for performance, so it's not really a good option.
Can anyone help? Thanks in advance!