There is a minute window for a race between put-fence removing the fence
and for a new transaction by an external party on the GTT mmap. That is
we must zap the mmap prior to removing the fence and not afterwards.

Fixes regression from
commit 61050808
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Tue Apr 17 15:31:31 2012 +0100

    drm/i915: Refactor put_fence() to use the common fence writing routine

v2: Remember the fence to remove with a local variable (gcc)
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>