drm/i915: convert some gem structures to per-ring V2

The active list and request list move into the ringbuffer structure,
so each can track its active objects in the order they are in that
ring.  The flushing list does not, as it doesn't matter which ring
caused data to end up in the render cache.  Objects gain a pointer to
the ring they are active on (if any).

Signed-off-by: Zou Nan hai <nanhai.zou@intel.com>
Signed-off-by: Xiang Hai hao <haihao.xiang@intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>