blue: Apply gamma curve to backlight

Currently the backlight brightness is a linear
function of the commanded brightness - but
human vision is not linear.  To compensate
for this we apply a gamma curve to the backlight
brightness.  This makes the perceived brightness
more linear in relation to the brightness settings.

TODO: Determine what effect this has on the
power profile estimates and if we care about
the innacuracies in screen power consumption
reporting...  power_profile estimates are
pretty unreliable as it is.

Change-Id: Ib14fe7752ae3617cb9da9c5ddb3e7f52b1db5e05