drm/i915: add audio power domain

This way the code is simpler and can also be used for other platforms
where the audio power domain->power well mapping is different.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Paulo Zanoni <paulo.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>