drm_hwcomposer: Support assigning planes in ValidateDisplay

In order to assign planes to layers in ValidateDisplay, testing
compositing with a DRM atomic modeset test is needed as PresentDisplay
is too late. This means most of PresentDisplay needs to be run from
ValidateDisplay, so refactor PresentDisplay to a common function adding
a test only option.

Signed-off-by: Rob Herring <robh@kernel.org>
diff --git a/drmhwctwo.h b/drmhwctwo.h
index aa9de3f..82a9768 100644
--- a/drmhwctwo.h
+++ b/drmhwctwo.h
@@ -185,6 +185,7 @@
     }
 
    private:
+    HWC2::Error CreateComposition(bool test);
     void AddFenceToRetireFence(int fd);
 
     DrmResources *drm_;