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

And a couple of miscellaneous cleanups to the main body of the IRQ loop;
move per-loop condition variables within the scope of the loop and move
the old DRI1 breadcrumb to the tail of the function and so only execute
it once.

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>