[laptop-mode] Make intel-hda powersaving configurable only when on battery

[Frederik Himpe]

Is it possible to make intel-hda powersaving to become active only when
the system is running on battery power in the next laptop-mode version?
In spite of what is said in the configuration file, there is a huge
disadvantage to intel-hda powersaving: it creates an annoying "plopping"
in the speakers when the sound card switches between active and passive
state. For example, when browsing the web, just hitting a page with
Flash (even if there's no sound) is enough to let the sound card activate
and cause a "plop". So it would be nice to only let this happen when
running on battery power.

-- 
Frederik Himpe