The ring will emit too many if semaphores are disabled since we do not
add the correct number to num_dwords anymore.

This was introduced:
commit 52ed2325
Author: Ben Widawsky <benjamin.widawsky@intel.com>
Date:   Mon Dec 16 20:50:38 2013 -0800

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

FWIW, the bug was fixed later in the series.

/me hangs head in shame.

Daniel: Also note that we should have merged the read-only semaphore
modparam before this patch.
Reported-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>