drm/i915/ringbuffer: Clear semaphore sync registers on ring init

Ensure that the sync registers are cleared every time we restart the
ring to avoid stale values from creeping in from random neutrinos.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=108888Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181203113701.12106-3-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -529,6 +529,13 @@ static int init_ring_common(struct intel_engine_cs *engine)
 
 	intel_engine_reset_breadcrumbs(engine);
 
+	if (HAS_LEGACY_SEMAPHORES(engine->i915)) {
+		I915_WRITE(RING_SYNC_0(engine->mmio_base), 0);
+		I915_WRITE(RING_SYNC_1(engine->mmio_base), 0);
+		if (HAS_VEBOX(dev_priv))
+			I915_WRITE(RING_SYNC_2(engine->mmio_base), 0);
+	}
+
 	/* Enforce ordering by reading HEAD register back */
 	I915_READ_HEAD(engine);