drm_hwcomposer: Disable unused planes

Right before queuing up a composition, go through the
list of unused planes and add disable markers such
that they don't remain active when the new frame is
posted.

BUG=chrome-os-parter:42311
TEST=Tested on smaug, turned on/off bunch of times, no dup icons

Change-Id: Ic2e5e210873efb6dc41fd43682fe00db33c2a28e
Signed-off-by: Sean Paul <seanpaul@chromium.org>
diff --git a/drmcomposition.h b/drmcomposition.h
index 902d7c7..06af71d 100644
--- a/drmcomposition.h
+++ b/drmcomposition.h
@@ -43,7 +43,10 @@
   virtual int AddLayer(int display, hwc_layer_1_t *layer, hwc_drm_bo_t *bo);
   int AddDpmsMode(int display, uint32_t dpms_mode);
 
+  int DisableUnusedPlanes();
+
   std::unique_ptr<DrmDisplayComposition> TakeDisplayComposition(int display);
+  DrmDisplayComposition *GetDisplayComposition(int display);
 
  private:
   DrmComposition(const DrmComposition &) = delete;