drm/i915: Enabling DebugFS for valleyview forcewake counts

Forcewake counts for valleyview are not exposed throgh DebugFS.
Exposing with this change.

Signed-off-by: Deepak S <deepak.s@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>