drm/i915: Remove gen4 irq code from gen3 irq routine

A couple of miscellaneous cleanups as well to move per-loop condition
variables within the scope of the loop and the update of the DRI1
breadcrumb to the tail of the function.

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>