drm/i915: fix FP CB tuning limits for lvds

Only on IBX should we set the limiting factor to 25 unconditionally
for dual-channel mode, on CPT/PPT 25 only applies when the lvds
refclock is 100MHz.

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