drm/i915: Assert the drm_mm_node is allocated when on the VM lists

Before moving the vma between the VM active/inactive lists, assert that
the node is still allocated.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170119192659.31789-5-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>