drm/i915: VLV/CHV PSR debugfs.

Add debugfs support for Valleyview and Cherryview considering that
we have PSR per pipe and  we don't have any kind of
performance counter as we have on other platforms that support PSR.

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Durgadoss R <durgadoss.r@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>