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

[Bart Samwel]

Hi Frederik,

Frederik Himpe wrote:
> 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.

Sounds fair enough -- if there is a downside to the power saving mode,
then we should allow switching based on AC / battery. I'll be sure to
pick this up in the next release!

Cheers,
Bart