drm/i915: Fix retrieval of hangcheck stats

The default context is always supported (as it contains the global
hangcheck stats) and the contexts for hangcheck are not limited
to any ring.

References: https://bugs.freedesktop.org/show_bug.cgi?id=65845


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>