drm/i915: Reset forcewake before suspend

Now that we regularly defer the forcewake dance to a timer func, it is
likely to fire after we disable the device during suspend. This
generates an oops as we detect inconsistency in the hardware state. So
before suspend, we want to complete the outstanding dance and generally
sanitize the registers before handing back to the BIOS.

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