drm/i915: Track active vma requests

Hook the vma itself into the i915_gem_request_retire() so that we can
accurately track when a solitary vma is inactive (as opposed to having
to wait for the entire object to be idle). This improves the interaction
when using multiple contexts (with full-ppgtt) and eliminates some
frequent list walking when retiring objects after a completed request.

A side-effect is that we get an active vma reference for free. The
consequence of this is shown in the next patch...

v2: Update inline names to be consistent with
i915_gem_object_get_active()

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470293567-10811-25-git-send-email-chris@chris-wilson.co.uk