drm/i915: Reduce i915_irq.c diffs with Linux 3.8.13

* i915_handle_rps_change() renamed to ironlake_handle_rps_change()

* pch_irq_handler() replaced by ibx_irq_handler() and cpt_irq_handler()

* Make ironlake_irq_handler() more similar to its Linux 3.8.13 version

* Make ivybridge_irq_handler() more similar to its Linux 3.8.13 version