drm/i915: Fix disabled semaphores

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>

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -673,10 +673,12 @@ gen6_add_request(struct intel_ring_buffer *ring)
 	if (ret)
 		return ret;
 
-	for_each_ring(useless, dev_priv, i) {
-		u32 mbox_reg = ring->signal_mbox[i];
-		if (mbox_reg != GEN6_NOSYNC)
-			update_mboxes(ring, mbox_reg);
+	if (i915_semaphore_is_enabled(dev)) {
+		for_each_ring(useless, dev_priv, i) {
+			u32 mbox_reg = ring->signal_mbox[i];
+			if (mbox_reg != GEN6_NOSYNC)
+				update_mboxes(ring, mbox_reg);
+		}
 	}
 
 	intel_ring_emit(ring, MI_STORE_DWORD_INDEX);