drm/i915: check whether the pch is the soulmate of the cpu

We don't really support fancy north display/pch combinations, so
put a big yelling WARN_ON in there. It /should/ be impossible, but
alas, the rumours don't stop (mostly due to really early silicon
sometimes using older PCHs).

v2: Fixup the logic fumble noticed by Paulo Zanoni. I should actually
try to test run the patch next time around ...

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