debugger: fix chain of fallbacks for disassembler view.

Use non-mixed mode if source is not available.

src/plugins/debugger/debuggeragents.h

@@ -86,10 +86,12 @@ public:
     void resetLocation();
     Q_SLOT void setContents(const QString &contents);
     QString address() const;
+    bool contentsCoversAddress(const QString &contents) const;
     void cleanup();
 
 private:
     DisassemblerViewAgentPrivate *d;
+    DebuggerManager *m_manager;
 };