drm/i915: Assert that we have allocated the drm_mm_node upon pinning

We currently check after the slow path that the vma is bound correctly,
but we don't currently check after the fast path. This is important in
case we accidentally take the fast path and leave the vma misplaced.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170111210937.29252-27-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>