In mesa/gallium, a pipe_fence can outlive the pipe_context it was
created from.  But to wait on the fence we need to know the submit-
queue (ie. the fd_pipe).

The most straightforward way to fix this is to add reference counting
to the fd_pipe and let the fence hold a reference to the pipe (rather
than hanging on to the context, which might have been destroyed before
the fence).

Signed-off-by: Rob Clark <robclark@freedesktop.org>