https://bugs.freedesktop.org/show_bug.cgi?id=81382
--- Comment #11 from Patrick ps_mail@freenet.de --- I've got the same notebook as Daniel Kirsten, so I've searched for changes in the kernel code, which caused the dark screen. From that point, I've found the patch from Alex Deucher and this bug report.
If I change /sys/class/backlight/radeon_bl0/brightness or /sys/class/backlight/acpi_video0 the display stayes dark.
@Alex is this what you wanted to know? If not, please let me know, how I could help you to debug this problem!