drm/i915: Assert that the handle->vma lut is empty on object close

Make sure that we are not leaking an entry in the ctx->handles_lut by
asserting that the object was removed prior to being freed. This should
be enforced by all such handles being removed by i915_gem_close_object.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20170822110517.22277-2-chris@chris-wilson.co.uk


Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>