drm/i915: Cleanup gen3 irq uninstall

Bring the for-each-pipe loops together so that the code is easier on the
eyes.

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>