drm/i915: unbreak i915_gem_object_ggtt_unbind()

There is an extra semi-colon here so we just leak and never unbind
anything.

This regression has been introduced in

commit 07fe0b12800d4752d729d4122c01f41f80a5ba5a
Author: Ben Widawsky <ben@bwidawsk.net>
Date:   Wed Jul 31 17:00:10 2013 -0700

    drm/i915: plumb VM into bind/unbind code

Cc: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>