drm/i915: Move unload time display power domain uninit later

Move the power domain uninitialization later so that it matches its
corresponding init order. Since we access the HW during the later
unitialization steps keep a wake reference until after the last such
step.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1458128348-15730-12-git-send-email-imre.deak@intel.com