Required so that we don't obliterate the queue if initialising the
rings after the global IRQ handler is installed.

[Jesse, you recently looked at refactoring the IRQ installation
routines, does moving the initialisation of ring buffer data structures away
from that routine make sense in your grand scheme?]
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Keith Packard <keithp@keithp.com>