anv: Add FIXME for vkResetCommandPool

anv: Add FIXME for vkResetCommandPool

vkResetCommandPool currently destroys its command buffers. The Vulkan
1.0 spec requires that it only reset them:

    Resetting a command pool recycles all of the resources from all of
    the command buffers allocated from the command pool back to the
    command pool. All command buffers that have been allocated from the
    command pool are put in the initial state.