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.h b/backend/BackendClient.h
index 13543f1..95abb0f 100644
--- a/backend/BackendClient.h
+++ b/backend/BackendClient.h
@@ -23,8 +23,7 @@
 
 class BackendClient : public Backend {
  public:
-  HWC2::Error ValidateDisplay(DrmHwcTwo::HwcDisplay *display,
-                              uint32_t *num_types,
+  HWC2::Error ValidateDisplay(HwcDisplay *display, uint32_t *num_types,
                               uint32_t *num_requests) override;
 };
 }  // namespace android