drm/i915: Missed dropped VMA conversion

This belonged in
commit 07fe0b12
Author: Ben Widawsky <ben@bwidawsk.net>
Date:   Wed Jul 31 17:00:10 2013 -0700

    drm/i915: plumb VM into bind/unbind code

But it was somehow missed along the way.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>