drm/i915: reuse the sdvo tv clock adjustment in ilk mode_set

Jesse extracted this nice helper in his i9xx_crtc_mode_set refactor,
but we have the identical code in ironlake_ccrtc_mode_set. And that
function is huge, so extracting some code full of magic numbers is
always nice.

Noticed while trying to get a handle on our dp clock code.

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