drm/i915: Free hardware status page on unload when physically mapped

A physically mapped hardware status page is allocated at driver load
time but was never freed. Call the existing code to free this page at
driver unload time on hardware which uses this kind.

Signed-off-by: Keith Packard <keithp@keithp.com>
[ickle: call before tearing down registers on KMS-only path, as pointed
out by Dave Airlie]
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@kernel.org