drm/i915: Consolidate forcewake resetting to a single function

We have two paths that try to reset the forcewake registers back to
known good values, with slightly different semantics and levels of
paranoia. Combine the two by passing a parameter to either restore the
forcewake status or to clear our bookkeeping, and raise the paranoia
level to max.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>