drm/i915: Implement WaSwitchSolVfFArbitrationPriority

Bspec mentions this for HSW+. I can't quite tell what the effects are,
and I don't easily have a way to test this.

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