drm/i915: avoid reading non-existent PLL reg on Ironlake+

These functions need to be reworked for Ironlake and above, but until
then at least avoid reading non-existent registers.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[ickle: combine with a gratuitous tidy]
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>