drm/i915: implement Disable4x2SubspanOptimization w/a for ivb, too

Copy&pasted from the vlv setup code. According to docs, we need that
on ivb, too.

v2: Use new masked bit handling macros.

Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>