https://bugzilla.kernel.org/show_bug.cgi?id=88501
--- Comment #5 from business.kid@gmail.com --- <Does booting a recent kernel with radeon.backlight=0 fix the issue?> Yes, this fixes the issue on 3.18-rc4
<Does this patch fix the issue (against 3.17 or newer kernel)?> No, it does not. I get the same behaviour - first time it blanks to dark, every other time it blanks to white.
I am a very rusty amateur at C, but I will adjust that patch very slightly here and have another go.