drm/i915: Don't emit mbox updates without semaphores

Aside from the fact that it leaves confusing dumps on error capture, it
is entirely unnecessary, and potentially harmful in cases like BDW,
where the instruction has changed.

In reality (seemingly), this will have no behavioral impact.

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