I always have to press by Fn + Brightness increase key in Ubuntu. I have switched off all the power management and still this happens. I am on a Lenovo ideapad U260 and can't to seem to set the brightness to same constant (high) at all times.
You can script this with xbacklight:
sudo apt-get install xbacklight
xbacklight -set 100 will set your brightness to 100%. You can add this as a new startup application (Search for 'Startup Applications', or I believe it's accessible by clicking the icon in the top right corner), or add this command to
/etc/rc.local if you want it to happen earlier in the boot sequence