When I switch to integrated intel graphics with:
#echo "DIGD" > /sys/kernel/debug/vgaswitcheroo/switch
logout & logback in with only laptop's screen on (obviously),
I _can_ control the laptop screen's 'brightness'. But,
when I switch to discrete Radeon HD 5650 graphics with:
#echo "DDIS" > /sys/kernel/debug/vgaswitcheroo/switch
logout & logback in with both laptop's screen and external monitor 'ON' - laptop screen's brightness becomes full, and I can't decrease the brightness!

I tried:
#echo 1000 > /sys/class/backlight/intel_backlight/brightness
but it doesn't show any effect on brightness of laptop's screen.

Also, the value in /sys/class/backlight/intel_backlight/brightness "jumps" to arbitrary number when I "echo" number into it to "lower" the brightness.

e.g. when I do:
#cd /sys/class/backlight/intel_backlight
#echo 4882 > brightness
the value in 'brightness' file jumps to 13000 and even 64332 or something on second try to lower the brightness.

Finally, let me know if any sort of logs or information is required...