If we retire requests last, we may use a later seqno and so clear
the requests lists without clearing the active list, leading to
confusion. Hence we should retire requests first for consistency with
the early return. The order used to be important as the lifecycle for
the object on the active list was determined by request->seqno. However,
the requests themselves are now reference counted removing the
constraint from the order of retirement.

Fixes regression from

commit 1b5a433a
Author: John Harrison <John.C.Harrison@Intel.com>
Date:   Mon Nov 24 18:49:42 2014 +0000

    drm/i915: Convert 'i915_seqno_passed' calls into 'i915_gem_request_completed
'

and a

	WARNING: CPU: 0 PID: 1383 at drivers/gpu/drm/i915/i915_gem_evict.c:279 i915_gem_evict_vm+0x10c/0x140()
	WARN_ON(!list_empty(&vm->active_list))

Identified by updating WATCH_LISTS:

	[drm:i915_verify_lists] *ERROR* blitter ring: active list not empty, but no requests
	WARNING: CPU: 0 PID: 681 at drivers/gpu/drm/i915/i915_gem.c:2751 i915_gem_retire_requests_ring+0x149/0x230()
	WARN_ON(i915_verify_lists(ring->dev))

Note that this is only a problem in evict_vm where the following happens
after a retire_request has cleaned out all requests, but not all active
bo:
- intel_ring_idle called from i915_gpu_idle notices that no requests are
  outstanding and immediately returns.
- i915_gem_retire_requests_ring called from i915_gem_retire_requests also
  immediately returns when there's no request, still leaving the bo on the
  active list.
- evict_vm hits the WARN_ON(!list_empty(&vm->active_list)) after evicting
  all active objects that there's still stuff left that shouldn't be
  there.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Cc: John Harrison <John.C.Harrison@Intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>