drm/i915/bdw: debugfs updates

All the gen8 debugfs stuff I wasn't too lazy to update. We'll need more
later, I am certain.

v2: Fix up the register name in the debugfs output as suggested by
Paulo.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net> (v1)
Reviewed-by: Paulo Zanoni <przanoni@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>