drm/i915: announce to userspace that the bsd ring is coherent

Otherwise we can't really fix the abi-braindeadness of forcing
libva to manually wait for rendering when switching rings. Which
in turn makes implementing hw semaphores a pointless exercise
(at least for ironlake).

[Also added the relaxed fencing param to explain the jump in
numbering - relaxed fencing is in -next.]

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>