drm/i915: introduce i915_hangcheck_ring_hung

In preparation to track per ring progress in hangcheck,
add i915_hangcheck_ring_hung.

v2: omit dev parameter (Ben Widawsky)

Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>