drm/i915: remove device field from struct power_well

The only real need for this field was in
i915_{request,release}_power_well, but there we can get at it by a
container_of magic. Also since in the future we'll have multiple power
wells each with its own power_well struct it makes sense to remove the
field from there where it'd be just redundancy.

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