drm/i915: use render gen to switch ring irq functions

Top-level interrupt bits are usually found in the display block. It
therefore makes sense to use HAS_PCH_SPLIT in i915_irq.c

But the irq stuff in intel_ring.c only concerns itself with render
core/gt-level interrupt sources. It therefore makes more sense to
switch based on gpu gen.

Kills a vlv special case.

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>