drm_hwcomposer: Set return type to std::optional for BufferInfoGetters

This is a bit of code modernization. Further changes will require indication
that buffer_info is valid, and using std::optional is the most correct
approach to do that.

Signed-off-by: Roman Stratiienko <roman.o.stratiienko@globallogic.com>
diff --git a/bufferinfo/legacy/BufferInfoImagination.h b/bufferinfo/legacy/BufferInfoImagination.h
index 4066d11..635e3b5 100644
--- a/bufferinfo/legacy/BufferInfoImagination.h
+++ b/bufferinfo/legacy/BufferInfoImagination.h
@@ -27,7 +27,7 @@
  public:
   using LegacyBufferInfoGetter::LegacyBufferInfoGetter;
 
-  int ConvertBoInfo(buffer_handle_t handle, BufferInfo *bo) override;
+  auto GetBoInfo(buffer_handle_t handle) -> std::optional<BufferInfo> override;
 };
 }  // namespace android