drm/i915: Assert we do not attempt to reuse an allocated node

i915_gem_gtt_reserve() and i915_gem_gtt_insert() can only work on
unallocated nodes. Check that the callers complies.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170115172740.28995-1-chris@chris-wilson.co.uk


Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>