adapters: Fix UnityDisplayPowerControl::turn_on memory leak

Created by: afrantzis

Fix a memory leak introduced by https://github.com/ubports/repowerd/commit/7689b45b69e53566eb4a48f27cd318de8bd0b724.

g_dbus_connection_call_sync() returns a g_variant which needs to be unreferenced to release its memory.

Fixes #1 (closed)