drm_hwcomposer: Move HwcDisplay out of DrmHwcTwo class

Reduces code complexity.

Closes: https://gitlab.freedesktop.org/drm-hwcomposer/drm-hwcomposer/-/issues/35
Signed-off-by: Roman Stratiienko <roman.o.stratiienko@globallogic.com>
diff --git a/backend/BackendClient.cpp b/backend/BackendClient.cpp
index 73a3b3d..606dca2 100644
--- a/backend/BackendClient.cpp
+++ b/backend/BackendClient.cpp
@@ -20,10 +20,10 @@
 
 namespace android {
 
-HWC2::Error BackendClient::ValidateDisplay(DrmHwcTwo::HwcDisplay *display,
+HWC2::Error BackendClient::ValidateDisplay(HwcDisplay *display,
                                            uint32_t *num_types,
                                            uint32_t * /*num_requests*/) {
-  for (auto & [ layer_handle, layer ] : display->layers()) {
+  for (auto &[layer_handle, layer] : display->layers()) {
     layer.SetValidatedType(HWC2::Composition::Client);
     ++*num_types;
   }