drm/i915: Fix retrieval of hangcheck stats

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.

This was dropped in 2013 because it was supposed to have been included
with Ben's full-ppgtt patch set. It never landed and the bug remains.

References: https://bugs.freedesktop.org/show_bug.cgi?id=65845
Link: http://patchwork.freedesktop.org/patch/msgid/1372175222-27622-1-git-send-email-mika.kuoppala@intel.com
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170616132849.29597-1-chris@chris-wilson.co.uk