Comment 6 for bug 1055231

This problem does also occur in Ubuntu Quantal 3.5.0-15.22. I've isolated this commit as the apparent cause of the problem:

  d56e635 drm/i915: don't forget the PCH backlight registers

This one isn't actually necessary for Sputnik backlight functionality at all, so I'll remove it from the Sputnik PPA kernel.

Due to additional problems observed with the whole patch set, we're going to remove them from the Ubuntu Quantal kernel as well, for the time being.