drm/i915: pending_flip_is_done is gen3, name it so

And remove the cargo-culted copy from the valleyview irq handler.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>