drm_hwcomposer: Added hotplug support of the external display

Unplug of the main display will not work because of
Activity Manager code(ActivityStackSupervisor.java:handleDisplayRemoved).
Only one display can be connected as an external
display (see SurfaceFlinger::determineDisplayType).

Tested-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Andrii Chepurnyi <andrii_chepurnyi@epam.com>
diff --git a/drmdisplaycompositor.h b/drmdisplaycompositor.h
index 67f6334..1005598 100644
--- a/drmdisplaycompositor.h
+++ b/drmdisplaycompositor.h
@@ -55,6 +55,7 @@
   int Composite();
   void Dump(std::ostringstream *out) const;
   void Vsync(int display, int64_t timestamp);
+  void ClearDisplay();
 
   std::tuple<uint32_t, uint32_t, int> GetActiveModeResolution();
 
@@ -82,7 +83,6 @@
   int ApplyDpms(DrmDisplayComposition *display_comp);
   int DisablePlanes(DrmDisplayComposition *display_comp);
 
-  void ClearDisplay();
   void ApplyFrame(std::unique_ptr<DrmDisplayComposition> composition,
                   int status, bool writeback = false);
   int FlattenActiveComposition();