drm/i915/bdw: Add BDW support in the i915 debugfs entry

The patch introduces fixes for the debugfs attributes emitted by
the i915 driver for GEN8. Currently, it is not emitting the correct
 attributes which include the status of RC6 states.

Change-Id: Ib2068a0cac9a5wq3f228e547fa1a097ad369d242df
Signed-off-by: Vedang Patel <vedang.patel@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>