drm_hwcomposer: Handle primary display in GetDisplayConnectionType()

Primary display should always be internal.

Closes: https://gitlab.freedesktop.org/drm-hwcomposer/drm-hwcomposer/-/issues/58
Signed-off-by: Roman Stratiienko <roman.o.stratiienko@globallogic.com>
diff --git a/hwc2_device/HwcDisplay.h b/hwc2_device/HwcDisplay.h
index c3e0f6e..76456b7 100644
--- a/hwc2_device/HwcDisplay.h
+++ b/hwc2_device/HwcDisplay.h
@@ -33,6 +33,8 @@
 class Backend;
 class DrmHwcTwo;
 
+inline constexpr uint32_t kPrimaryDisplay = 0;
+
 class HwcDisplay {
  public:
   HwcDisplay(ResourceManager *resource_manager, DrmDevice *drm,
@@ -207,7 +209,8 @@
    * https://source.android.com/devices/graphics/hotplug#handling-common-scenarios
    */
   bool IsInHeadlessMode() {
-    return handle_ == 0 && connector_->state() != DRM_MODE_CONNECTED;
+    return handle_ == kPrimaryDisplay &&
+           connector_->state() != DRM_MODE_CONNECTED;
   }
 
  private: