Merge lp://staging/~jderose/system76-driver/fix-1378974 into lp://staging/system76-driver

For more background, please see:
https://bugs.launchpad.net/system76-driver/+bug/1378974

"start on (login-session-start or desktop-session-start)" seems to do the trick... I didn't experience a single brightness restore failures when testing with this modified Upstart job over the weekend, whereas the current system76-driver daily build for Utopic fails to restore the brightness very frequently (certainly over 50% of the time, at least on Intel GPU systems).

Looking at /etc/init/lightdm.conf more closely, I'd say this new Upstart job is generally more correct regardless of this specific bug, but the important thing is this change has proven to fix our problem.

I'm not going to dig into what changes in lightdm, etc., might have caused this regression, as there are many other higher priority bugs I still need to fry.