drm/i915/bdw: Implement WaSwitchSolVfFArbitrationPriority

GEN8 also needs this workaround.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
[danvet: Add a generic comment that we need to recheck all these w/a.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>