[dxvk] Use waitForIdle when destroying DXVK device

Otherwise, we might call vkDeviceWaitIdle before all command buffers
have been submitted to the Vulkan queue. Found by inspection.

src/dxvk/dxvk_device.cpp

@@ -30,7 +30,7 @@ namespace dxvk {
   DxvkDevice::~DxvkDevice() {
     // Wait for all pending Vulkan commands to be
     // executed before we destroy any resources.
-    m_vkd->vkDeviceWaitIdle(m_vkd->device());
+    this->waitForIdle();
   }