drm/i915/contexts: fix list corruption

After reset we unconditionally reinitialize lists. If the context switch
hasn't yet completed before the suspend, the default context object will
end up on lists that are going to go away when we resume.

The patch forces the context switch to be synchronous before suspend
assuring that the active/inactive tracking is correct at the time of
resume.

References: https://bugs.freedesktop.org/show_bug.cgi?id=52429
Tested-by: Guang A Yang <guang.a.yang@intel.com>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem.c

@@ -2365,17 +2365,17 @@ int i915_gpu_idle(struct drm_device *dev)
 
 	/* Flush everything onto the inactive list. */
 	for_each_ring(ring, dev_priv, i) {
+		ret = i915_switch_context(ring, NULL, DEFAULT_CONTEXT_ID);
+		if (ret)
+			return ret;
+
 		ret = i915_ring_idle(ring);
 		if (ret)
 			return ret;
 
 		/* Is the device fubar? */
 		if (WARN_ON(!list_empty(&ring->gpu_write_list)))
 			return -EBUSY;
-
-		ret = i915_switch_context(ring, NULL, DEFAULT_CONTEXT_ID);
-		if (ret)
-			return ret;
 	}
 
 	return 0;