drm/i915: Document the multi-threaded FORCEWAKE bits

No functional change, but reserves 0x2 for use by userspace.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>