The following commands assume that Arch Linux is installed, they should be applicable to any systemd based distro whatsoever.
# install ddccontrol or gddccontrol if you wish a GUI
$ pacman -S ddccontrol
# insert the required kernel module
$ modprobe i2c-dev
# make sure that the i2c-dev kernel module is inserted automatically on boot
$ cat <<HEREDOC> /etc/modules-load.d/i2c-dev.conf
# Load i2c-dev module for ddccontrol
i2c-dev
HEREDOC