drm/i915: Only check for valid PP_{ON, OFF}_DELAYS on pre ILK hardware

ILK+ have this register on the PCH. This check was triggering unclaimed
writes.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>