vulkan/pipeline_cache: Load raw objects from disk properly

When caching NIR, it's cached as a raw object because we cache the serialized NIR. When it's then loaded from the disk cache later, we fail to deserialize it because raw objects are a special case. We need to handle raw objects ourselves in the disk cache loading code.

Fixes: 591da987 ("vulkan: Add a common VkPipelineCache implementation")