drm/i915: Move per ring error state to ring_error

v2: Moved num_requests up (Chris)
Rebased on new hws page capture which required a rename since it made
two members named, 'hws' in the per ring error state. (Ben)

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>