drm/i915: Remove the conflicting BUG_ON()

We now attempt to free "active" objects following a GPU hang as either
the GPU will be reset or the hang is permenant. In either case, the GPU
writes will not be flushed to main memory and it should be safe to
return that memory back to the system.

The BUG_ON(active) is thus overkill and can erroneously fire after a
EIO.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NEric Anholt <eric@anholt.net>

drivers/gpu/drm/i915/i915_gem.c

@@ -1979,8 +1979,6 @@ i915_gem_object_unbind(struct drm_gem_object *obj)
 	 * cause memory corruption through use-after-free.
 	 */
 
-	BUG_ON(obj_priv->active);
-
 	/* release the fence reg _after_ flushing */
 	if (obj_priv->fence_reg != I915_FENCE_REG_NONE)
 		i915_gem_clear_fence_reg(obj);