drm/i915: Assert the vma's active tracking is clear before free

In looking at a use-after-free on Baytrail, it looks like the VMA's
activity tracking is suspect. Add some asserts to catch freeing the VMA
before we have decoupled all of its i915_gem_active trackers.

References: https://bugs.freedesktop.org/show_bug.cgi?id=101511


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170620124321.1108-3-chris@chris-wilson.co.uk


Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>