drm/i915: prepare for multiple power wells

In the future we'll need to support multiple power wells, so prepare for
that here. Create a new power domains struct which contains all
power domain/well specific fields. Since we'll have one lock protecting
all power wells, move power_well->lock to the new struct too.

No functional change.

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>