drm/i915: Disable hangcheck when wedged

If the gpu reset fails and the machine is terminally wedged, further
hangchecks achieve nothing but noise. Disable them, with a corollary
that we re-enable hangchecking after a successful GPU reset in case the
user is artificially bringing the machine back to life through the debug
interface.

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/20161122144121.7379-2-chris@chris-wilson.co.uk