drm/i915: Don't overclock on Haswell

HSW doesn't overclock the same way as IVB or SNB. I do not know about
VLV, so I've kept that off as well. I'm still working on getting the doc
updates to explain how we overclock on Haswell.

Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[danvet: Add missing () spotted by Wu Fengguang's kernel build robot.
Acked by Ben.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>