hwc: Add support for Secure display

- Identify secure display layer in the hwc_list
- Need to set both SECURE_OVERLAY and SECURE_DISPLAY flags for the secure
  display layer
- Disable idle timeout for secure display usecase, as GPU should not be
  accessing secure display layer

Change-Id: I555910db77c466b5e103b24b4f0ec7f47bb210a5

libgralloc/gpu.cpp

@@ -146,6 +146,10 @@ int gpu_context_t::gralloc_alloc_buffer(size_t size, int usage,
             flags |= private_handle_t::PRIV_FLAGS_HW_TEXTURE;
         }
 
+        if(usage & GRALLOC_USAGE_PRIVATE_SECURE_DISPLAY) {
+            flags |= private_handle_t::PRIV_FLAGS_SECURE_DISPLAY;
+        }
+
         flags |= data.allocType;
         int eBaseAddr = int(eData.base) + eData.offset;
         private_handle_t *hnd = new private_handle_t(data.fd, size, flags,