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.cpp b/libdrmutils/drm_master.cpp
index 239ec4c..e12b933 100644
--- a/libdrmutils/drm_master.cpp
+++ b/libdrmutils/drm_master.cpp
@@ -73,6 +73,12 @@
   return 0;
 }
 
+void DRMMaster::DestroyInstance() {
+  lock_guard<mutex> obj(s_lock);
+  delete s_instance;
+  s_instance = nullptr;
+}
+
 int DRMMaster::Init() {
   dev_fd_ = drmOpen("msm_drm", nullptr);
   if (dev_fd_ < 0) {