drm/i915: drop unnecessary check from fdi_link_train code

commit 8f5718a6d9a29bf1f51910c6b32bd0244f905303 upstream.

They are all written for a specific north disaplay->pch combination.
So stop pretending otherwise.

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>