drm_hwcomposer: Add headless mode support

Headless mode required to keep SurfaceFlinger alive when all displays are
disconnected, Without headless mode Android will continuously crash.
Only single internal (primary) display is required to be in HEADLESS mode
to prevent the crash. See [1].

[1]: https://source.android.com/devices/graphics/hotplug#handling-common-scenarios
Closes: https://gitlab.freedesktop.org/drm-hwcomposer/drm-hwcomposer/-/issues/57
Signed-off-by: Roman Stratiienko <roman.o.stratiienko@globallogic.com>
diff --git a/hwc2_device/HwcDisplayConfigs.h b/hwc2_device/HwcDisplayConfigs.h
index 7450b8d..5bcf696 100644
--- a/hwc2_device/HwcDisplayConfigs.h
+++ b/hwc2_device/HwcDisplayConfigs.h
@@ -47,6 +47,9 @@
   int preferred_config_id = 0;
 
   int last_config_id = 1;
+
+  uint32_t mm_width = 0;
+  uint32_t mm_height = 0;
 };
 
 }  // namespace android