drm/i915: Split the batch pool by engine

drm/i915: Split the batch pool by engine

I woke up one morning and found 50k objects sitting in the batch pool
and every search seemed to iterate the entire list... Painting the
screen in oils would provide a more fluid display.

One issue with the current design is that we only check for retirements
on the current ring when preparing to submit a new batch. This means
that we can have thousands of "active" batches on another ring that we
have to walk over. The simplest way to avoid that is to split the pools
per ring and then our LRU execution ordering will also ensure that the
inactive buffers remain at the front.

v2: execlists still requires duplicate code.
v3: execlists requires more duplicate code

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by:  Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>