vkd3d: Only put the mutable descriptor set once in the pipeline layout.

As a further optimization, the descriptor set allocation might be moved to the root signature creation, where decisions can be taken knowing exactly what is needed. For example, if there are not static samplers that descriptor set is not going to be used and might be free for push descriptors. It's even possible that moving set allocation even later, when the pipeline is created, we could take a hybrid approach in which some descriptors are statically bound and others are bound through dynamic indexing. I guess in some cases that might allow for a better optimization by the driver. Or maybe the driver is already able to do that, I don't know. That's just food for thought in the future.