drm/i915: IBX has a fixed pch pll to pch pipe mapping

This should fix breakage introduced in

commit ee7b9f93


Author: Jesse Barnes <jbarnes@virtuousgeek.org>
Date:   Fri Apr 20 17:11:53 2012 +0100

    drm/i915: manage PCH PLLs separately from pipes

v2: Add a DRM_DEBUG_KMS message to explain why a given pll was
selected, suggested by Chris Wilson.

v3: Actually run git add.

Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=49712
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>