[d3d11] Reset implicit flush timer only when actually flushing

In some situations, calling Flush without any commands that
could be flushed would unnecessarily prevent the implicit
flush heuristic from kicking in properly.

src/d3d11/d3d11_context_imm.cpp

@@ -103,11 +103,10 @@ namespace dxvk {
       
       FlushCsChunk();
       
-    }
-
-    // Reset optimization info
-    m_csIsBusy  = false;
+      // Reset flush timer used for implicit flushes
       m_lastFlush = std::chrono::high_resolution_clock::now();
+      m_csIsBusy  = false;
+    }
   }