The above patch causes a problem on my Amilo Xi 2550 (RadeonHD 2600).

The behaviour for kernel-3.14.8-100.fc19 (without patch):

The value of /sys/class/backlight/radeon_bl0/brightness is 0. 
Whenever I write something (0-255) to 
/sys/class/backlight/radeon_bl0/brightness, 
the backlight is turned off, and I am not able to turn on the backlight again. 
However, I can still shutdown the system (Ctr+Alt+Fx, login as root, and give a 
shutdown command.

I can control the backlight by writing values 0-7 to
/sys/class/backlight/acpi_video0/brightness.

The behaviour for kernel-3.14.17-100.fc19 (with patch):

The backlight is turned off during the boot up process, when the kernel
switches into 
graphics mode.    
I commented out the lines 233-234 in
drivers/gpu/drm/radeon/atombios_encoders.c, 
and then, the problem disappeared, i.e., the backlight stays on when the kernel
switches 
into graphics mode.

Best regards,
Daniel