drm_hwcomposer: Cursor plane binding

This change adds cursor plane binding into DrmDisplayPipeline.
The signature of DrmDisplayPipeline::GetUsablePlanes is updated
to return a pair where the second element points to the bound
cursor plane (if it exists), and the first element points to
all other planes.

Change-Id: Ic6a623b10383b7dd0a0c2e79fc2d87329ebc6214
Signed-off-by: Andrew Wolfers <aswolfers@google.com>
diff --git a/compositor/DrmKmsPlan.cpp b/compositor/DrmKmsPlan.cpp
index 4f75e89..443b515 100644
--- a/compositor/DrmKmsPlan.cpp
+++ b/compositor/DrmKmsPlan.cpp
@@ -28,7 +28,7 @@
     -> std::unique_ptr<DrmKmsPlan> {
   auto plan = std::make_unique<DrmKmsPlan>();
 
-  auto avail_planes = pipe.GetUsablePlanes();
+  auto [avail_planes, cursor_plane] = pipe.GetUsablePlanes();
 
   int z_pos = 0;
   for (auto &dhl : composition) {