venus: pre-initialize device groups

We don't need to worry about how vkEnumeratePhysicalDeviceGroups is
called (props is NULL, props is non-NULL but count is 0, etc.) this way.
It also allows us to fix up VkPhysicalDeviceGroupProperties easily.

v2: let the for-loop increment (Yiwei)

Signed-off-by: Chia-I Wu <olvaffe@gmail.com>
Reviewed-by: Yiwei Zhang <zzyiwei@chromium.org> (v1)
Reviewed-by: Ryan Neph <ryanneph@google.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12637>

src/virtio/vulkan/vn_instance.c

@@ -779,6 +779,7 @@ vn_DestroyInstance(VkInstance _instance,
       return;
 
    if (instance->physical_devices) {
+      vk_free(alloc, instance->physical_device_groups);
       for (uint32_t i = 0; i < instance->physical_device_count; i++)
          vn_physical_device_fini(&instance->physical_devices[i]);
       vk_free(alloc, instance->physical_devices);