drm/i915: Record DERRMR, FORCEWAKE and RING_CTL in error-state

These are useful for investigating hangs involving WAIT_FOR_EVENT.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
[danvet: Apply a droplet of Future-Proof in the if-ladder.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>