drm/i915: Show if we consider the engine is idle in the GPU error state

Useful for verifying our bookkeeper when we encounter is knowing whether
we think the engine is idle at the time of the GPU hang.

References: https://bugs.freedesktop.org/show_bug.cgi?id=104305
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171219131419.13117-1-chris@chris-wilson.co.uk