drm/i915: Stop capturing semaphore registers for gen6/7 GPU hangs

We no longer use the semaphore sync registers on gen6/7, so including
them in the GPU error state is mere noise.

References: 6faf5916 ("drm/i915: Remove HW semaphores for gen7 inter-engine synchronisation")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190305150914.11340-2-chris@chris-wilson.co.uk