drm/i915: add LynxPoint-LP PCH ID

[pzanoni: rebase, print it's an LP PCH]

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Tested-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>