Wait for all device work to finish before destroy VkDevice

Currently before destroying the GrVkGpu, we do wait on all queue work to finish before destroying objects there.
Since we only have one queue, the vkDeviceWaitIdle added here is probably not necessary, but the spec does suggest
having it just to make sure all work is fully done on the device.

BUG=skia:
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Review URL: https://codereview.chromium.org/1876863004

src/gpu/vk/GrVkBackendContext.cpp

@@ -238,6 +238,7 @@ const GrVkBackendContext* GrVkBackendContext::Create() {
 }
 
 GrVkBackendContext::~GrVkBackendContext() {
+    vkDeviceWaitIdle(fDevice);
     vkDestroyDevice(fDevice, nullptr);
     fDevice = VK_NULL_HANDLE;
     vkDestroyInstance(fInstance, nullptr);