drm/i915: add i915_pc8_status debugfs file

Make it print the value of the variables on the PC8 struct.

v2: Update to recent renames and add the new fields.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>