drm/i915: Ivybridge MI_ARB_ON_OFF context w/a

The workaround itself applies to gen7 only (according to the docs) and
as Eric Anholt points out shouldn't be required since we don't use HW
scheduling features, and therefore arbitration. Though since it is a
small, and simple addition, and we don't really understand the issue,
just do it.

FWIW, I eventually want to play with some of the arbitration stuff, and
I'd hate to forget about this.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>