drm/i915: fix ACTHD for gen <= 3

This was mixed up in the following patch:

commit a6c45cf0
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Fri Sep 17 00:32:17 2010 +0100

    drm/i915: INTEL_INFO->gen supercedes i8xx, i9xx, i965g

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>