I am affected by this bug too.
I have a Acer 3820TG laptop featuring Intel IGP (Ironlake) / AMD DGP (Radeon
6550M). It kept me stucked with kernel 3.12 series until I googled for
"backlight issue" rather than "Intel driver issue" and found THIS bug report.
:-)

adding radeon.runpm=0 to the boot options works for me too. I haven't tested
the patch yet.

FWIW, I'd like to add 2 side informations hoping it might be related :
- the backlight control only works for the Intel IGP. If the laptop goes idle
when on the radeon, waking it up doesn't bring the backlight back.
- when switching from Ironlake to Radeon through VGA switcheroo, I have to use
"ON -> MIGD -> DDIS" sequence as the screen goes black immediately after
passing "ON".