drm/i915: Only enable IPS polling for gen5

drm/i915: Only enable IPS polling for gen5

On SandyBridge IPS was entirely implemented in hardware and not reliant
on the driver monitoring power consumption and feeding back desired run
states, so the hardware is able to adapt quicker and more flexibly. Which
is a huge relief for us as we no longer have to carry empirically
derived magic algorithms.

Yet despite the advance in technology, the driver was still doing its
IPS polling on all machines. Restrict it to the only supported hardware,
Clarkdale/Arrandale.

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>