drm/i915: don't disable fdi links harder in ilk_crtc_enable

Because they should have been disabled when shutting down the display
pipe previously. To ensure that this is the case, add a few assserts
instead of unconditionally disabling the fdi link.

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>