We may call mlx4_dispatch_event() before mlx4_register_device() is
called for a device, because for example a catastrophic error happens
immediately after we enable interrupts.  Therefore priv->ctx_list and
priv->ctx_lock need to be initialized earlier.

This bug was actually exposed by the MSI-X bug that returned IRQ numbers 
to drivers in reverse order, so that the first FW command 
interrupt looked to mlx4 like a catastrophic error.
Signed-off-by: Roland Dreier <rolandd@cisco.com>