drm/i915: clean up unused ring_get_irq/ring_put_irq functions

This patch depends on patch "drm/i915: fix user irq miss in BSD ring on
g4x".
Once the previous patch apply, ring_get_irq/ring_put_irq become unused.
So simply remove them.

Signed-off-by: Feng, Boqun <boqun.feng@intel.com>
Reviewed-by: Xiang, Haihao <haihao.xiang@intel.com>
Signed-off-by: Keith Packard <keithp@keithp.com>