drm/i915: make the panel fitter work on pipes B and C on Haswell

This goes on a separate patch since it won't apply on the stable
trees and there's nothing using panel fitter on HSW on the older
Kernels.

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