drm/i915: unify ring irq refcounts (again)

With the simplified locking there's no reason any more to keep the
refcounts seperate.

v2: Readd the lost comment that ring->irq_refcount is protected by
dev_priv->irq_lock.

Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>