drm/i915/ringbuffer: Use the HEAD auto-reporting mechanism

My Sandybridge only reports 0 for the ring buffer registers, causing it
to hang as soon as we exhaust the available ring. As a workaround, take
advantage of our huge ring buffers and use the auto-reporting mechanism
to update the status page with the HEAD location every 64 KiB.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>