drm/i915: vlv: get power domain for eDP vdd

Besides D0 device state we need the proper power wells to be on on
some platforms, so get the port power domain reference instead of an RPM
reference.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Antti Koskipää <antti.koskipaa@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>