sdm: Destroy singleton DRMMaster instance

Destroy singleton DRMMaster instance on teardown, this is important
for back to back API test runs

Change-Id: I27e5d2adcc9d0805281dd0c858eaed5af745b269
CRs-fixed: 1114808
diff --git a/libdrmutils/drm_master.h b/libdrmutils/drm_master.h
index 8c32a1b..15fae68 100644
--- a/libdrmutils/drm_master.h
+++ b/libdrmutils/drm_master.h
@@ -79,6 +79,7 @@
    *   -ENODEV if device cannot be opened or initilization fails
    */
   static int GetInstance(DRMMaster **master);
+  static void DestroyInstance();
 
  private:
   DRMMaster() {}