i965/bufmgr: Set bo->idle after waiting.

After a successful wait, we know the buffer ought to be idle.

Chris points out that: "The only caveat here is that bo is global, and
we have a very unlikely (and probably unnoticeable) race condition with
multiple contexts."

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>

src/mesa/drivers/dri/i965/brw_bufmgr.c

@@ -1018,6 +1018,8 @@ brw_bo_wait(struct brw_bo *bo, int64_t timeout_ns)
    if (ret == -1)
       return -errno;
 
+   bo->idle = true;
+
    return ret;
 }