drm/i915: Future proof interrupt handler.

These functions are already being called for gen >= 9,
so let's be sure when this happens we use whatever is
there already for the latest platform.

No functional change.

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>