drm/i915: debugfs: show semaphore registers also on gen7

Corresponding changes to improve our error_state are pending
some other patches to clean up things first.

Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Keith Packard <keithp@keithp.com>