drm_hwcomposer: Introduce main lock

Further development will require more asynchronous processing.
Introduce project-wide single mutex for these purposes.
Use it instead for callback handling instead of callback_lock.

Signed-off-by: Roman Stratiienko <roman.o.stratiienko@globallogic.com>
diff --git a/hwc2_device/DrmHwcTwo.h b/hwc2_device/DrmHwcTwo.h
index d096160..f38ba05 100644
--- a/hwc2_device/DrmHwcTwo.h
+++ b/hwc2_device/DrmHwcTwo.h
@@ -37,8 +37,6 @@
 #endif
   std::pair<HWC2_PFN_REFRESH, hwc2_callback_data_t> refresh_callback_{};
 
-  std::mutex callback_lock_;
-
   static HwcDisplay *GetDisplay(DrmHwcTwo *hwc, hwc2_display_t display_handle) {
     auto it = hwc->displays_.find(display_handle);
     if (it == hwc->displays_.end())
@@ -57,6 +55,10 @@
                                hwc2_function_pointer_t function);
   HWC2::Error CreateDisplay(hwc2_display_t displ, HWC2::DisplayType type);
 
+  auto &GetResMan() {
+    return resource_manager_;
+  }
+
  private:
   void HandleDisplayHotplug(hwc2_display_t displayid, int state);
   void HandleInitialHotplugState(DrmDevice *drmDevice);