drm_hwcomposer: Use Planner interface to provision planes

Use the new Planner interface to handle the layer->plane mapping.
This allows us to simplify the Plan() function by offloading the
plane provisioning to the platform specific code.

BUG=b/28117135
TEST=Tested on ryu with a variety of window layouts/workloads

Signed-off-by: Sean Paul <seanpaul@chromium.org>
Change-Id: I75a0c5d87a9096e7a83ecbc848c75fee42ee1131
diff --git a/drmcompositor.h b/drmcompositor.h
index 27623d9..19271b5 100644
--- a/drmcompositor.h
+++ b/drmcompositor.h
@@ -44,6 +44,7 @@
   DrmCompositor(const DrmCompositor &) = delete;
 
   DrmResources *drm_;
+  std::unique_ptr<Planner> planner_;
 
   uint64_t frame_no_;