drm/i915: DE_PCU_EVENT irq is ilk-only

Like all the other drps/ips stuff. Hence add the corresponding check,
give the function a preciser prefix and move the single reg clearing into
the rps handling function, too.

Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>