drm/i915: Simplify calling engine->sync_to

Since requests can no longer be generated as a side-effect of
intel_ring_begin(), we know that the seqno will be unchanged during
ring-emission. This predicatablity then means we do not have to check
for the seqno wrapping around whilst emitting the semaphore for
engine->sync_to().

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1469432687-22756-31-git-send-email-chris@chris-wilson.co.uk
Link: http://patchwork.freedesktop.org/patch/msgid/1470174640-18242-22-git-send-email-chris@chris-wilson.co.uk
diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index d79b949..3df6b48 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -2867,22 +2867,15 @@
 		i915_gem_object_retire_request(obj, from);
 	} else {
 		int idx = intel_engine_sync_index(from->engine, to->engine);
-		u32 seqno = i915_gem_request_get_seqno(from);
-
-		if (seqno <= from->engine->semaphore.sync_seqno[idx])
+		if (from->fence.seqno <= from->engine->semaphore.sync_seqno[idx])
 			return 0;
 
 		trace_i915_gem_ring_sync_to(to, from);
-		ret = to->engine->semaphore.sync_to(to, from->engine, seqno);
+		ret = to->engine->semaphore.sync_to(to, from);
 		if (ret)
 			return ret;
 
-		/* We use last_read_req because sync_to()
-		 * might have just caused seqno wrap under
-		 * the radar.
-		 */
-		from->engine->semaphore.sync_seqno[idx] =
-			i915_gem_request_get_seqno(obj->last_read_req[from->engine->id]);
+		from->engine->semaphore.sync_seqno[idx] = from->fence.seqno;
 	}
 
 	return 0;