drm/i915: Always call fence-lost prior to removing the fence

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 61050808bb019ebea966b7b5bfd357aaf219fb51
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>

drivers/gpu/drm/i915/i915_gem.c

@@ -2128,11 +2128,11 @@ static void i915_gem_reset_fences(struct drm_device *dev)
 	for (i = 0; i < dev_priv->num_fence_regs; i++) {
 		struct drm_i915_fence_reg *reg = &dev_priv->fence_regs[i];
 
-		i915_gem_write_fence(dev, i, NULL);
-
 		if (reg->obj)
 			i915_gem_object_fence_lost(reg->obj);
 
+		i915_gem_write_fence(dev, i, NULL);
+
 		reg->pin_count = 0;
 		reg->obj = NULL;
 		INIT_LIST_HEAD(&reg->lru_list);
@@ -2722,6 +2722,7 @@ int
 i915_gem_object_put_fence(struct drm_i915_gem_object *obj)
 {
 	struct drm_i915_private *dev_priv = obj->base.dev->dev_private;
+	struct drm_i915_fence_reg *fence;
 	int ret;
 
 	ret = i915_gem_object_wait_fence(obj);
@@ -2731,10 +2732,10 @@ i915_gem_object_put_fence(struct drm_i915_gem_object *obj)
 	if (obj->fence_reg == I915_FENCE_REG_NONE)
 		return 0;
 
-	i915_gem_object_update_fence(obj,
-				     &dev_priv->fence_regs[obj->fence_reg],
-				     false);
+	fence = &dev_priv->fence_regs[obj->fence_reg];
+
 	i915_gem_object_fence_lost(obj);
+	i915_gem_object_update_fence(obj, fence, false);
 
 	return 0;
 }