[dxvk] Add adapterCount function to DXVKInstance.

Will be needed in the Direct3D9 interface as you can query the number of adapters.

src/dxvk/dxvk_instance.h

@@ -39,6 +39,15 @@ namespace dxvk {
       return m_vki->instance();
     }
 
+     /**
+     * \brief Number of adapters
+     * 
+     * \returns The number of adapters
+     */
+    size_t adapterCount() {
+      return m_adapters.size();
+    }
+    
     /**
      * \brief Retrieves an adapter
      *