drm/i915: enable intel_lvds->pre_pll_enable for ilk+, too

Only two things needed adjustment:
- pipe select for PCH_CPT
- There's no dithering bit on ilk+ in the lvds ctl reg

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