drm_hwcomposer: Make main mutex recursive

It allows to remove redundant unlock/lock pair from the code,
and should make it a little bit more race-proof.

Signed-off-by: Roman Stratiienko <r.stratiienko@gmail.com>
diff --git a/hwc2_device/DrmHwcTwo.h b/hwc2_device/DrmHwcTwo.h
index 7a65853..81c5155 100644
--- a/hwc2_device/DrmHwcTwo.h
+++ b/hwc2_device/DrmHwcTwo.h
@@ -71,7 +71,7 @@
                                                  int64_t timestamp) const;
 
  private:
-  void SendHotplugEventToClient(hwc2_display_t displayid, bool connected);
+  void SendHotplugEventToClient(hwc2_display_t displayid, bool connected) const;
 
   ResourceManager resource_manager_;
   std::map<hwc2_display_t, std::unique_ptr<HwcDisplay>> displays_;