Some keyboard brightness controllers have different ranges for levels.
Some have a few discreet values. Others are more like a percent. This
change uses a step based on the maximum level. If less than 20, use 1.
Else, divide by 20.

This logic is the same as used in gnome-settings-daemon. It comes from
the BRIGHTNESS_STEP_AMOUND macro in plugins/power/gpm-common.h.

Signed-off-by: Eric Koegel <eric.koegel@gmail.com>