[display] Disallow simultaneous use of wait events

Having multiple images simultaneous wait on the same event requires
defining interactions between different events and resolving race
conditions around events being reset. Synchronization between layers
can be achieved through other parts of the API, and creating/importing
new events isn't particularly expensive, so just disallow simultaneous
use of wait events.

This can be re-examined at a later point if a fence object is added to
fuchsia.

Test: locally modifications to display test
Change-Id: I0464f39165ef5f41fe401a26c4a90b2ef7e9400a

system/dev/display/display/client.cpp

@@ -734,8 +734,14 @@ void Client::HandleApplyConfig(const fuchsia_display_ControllerApplyConfigReques
             }
 
             if (layer->pending_image_) {
+                auto wait_fence = GetFence(layer->pending_wait_event_id_);
+                if (wait_fence && wait_fence->InContainer()) {
+                    zxlogf(ERROR, "Tried to wait with a busy event\n");
+                    TearDown();
+                    return;
+                }
                 layer_node.layer->pending_image_->PrepareFences(
-                        GetFence(layer->pending_wait_event_id_),
+                        fbl::move(wait_fence),
                         GetFence(layer->pending_signal_event_id_));
                 {
                     fbl::AutoLock lock(controller_->mtx());

system/fidl/fuchsia-display/display-controller.fidl

@@ -365,7 +365,8 @@ interface Controller {
     // call it on a color layer, or to call it with an image and layer whose
     // ImageConfigs do not match. It is illegal to apply a configuration
     // with an image layer that has no image (note that is is not illegal to
-    // validate such a configuration).
+    // validate such a configuration). It is illegal to reuse a wait event which
+    // another layer that has not been presented is waiting on.
     17: SetLayerImage(uint64 layer_id, uint64 image_id, uint64 wait_event_id, uint64 signal_event_id);
 
     // Attempts to validate the current configuration.