drm/i915: Fix oops on HWS unload

Freeing the Hardware Status Page was writing to the HWS register in
order to disable the GPU writing to the HWS page. Unfortunately, we were
writing to the mmio register after unmapping the register space, hence
the oops.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>