Implement unmap operation which destroys all mapping for a BO

Snippet of an email from @airlied and @jekstrand:

>> I suppose it also asks the question around paralleling
>>
>> fd = open()
>> ptr = mmap(fd,)
>> close(fd)
>> the mapping is still valid.
>>
>> I suppose our equiv is
>> handle = bo_alloc()
>> gpu_addr = vm_bind(handle,)
>> gem_close(handle)
>> is the gpu_addr still valid does the VM hold a reference on the kernel
>> bo internally.
> For Vulkan it looks like this is undefined and the above is not necessary:
>
> "It is important to note that freeing a VkDeviceMemory object with
> vkFreeMemory will not cause resources (or resource regions) bound to
> the memory object to become unbound. Applications must not access
> resources bound to memory that has been freed."
> (32.7.6)

I'm not sure about this particular question.  We need to be sure that maps get cleaned up eventually.  On the one hand, I think it's probably a valid API implementation to have each mapped page hold a reference similar to mmap and have vkDestroyImage or vkDestroyBuffer do an unmap to clean up the range.  However, clients may be surprised when they destroy a large memory object and can't reap the memory because of extra BO references they don't know about.  If BOs unmap themselves on close or if we had a way to take a VM+BO and say "unmap this BO from everywhere, please", we can clean up the memory pretty easily.  Without that, it's a giant PITA to do entirely inside the userspace driver because it requires us to globally track every mapping and that means data structures and locks.  Yes, such an ioctl would require the kernel to track things but the kernel is already tracking everything that's bound, so hopefully it doesn't add much.

--Jason

Thinking the uAPI changes would be: https://pastebin.com/vUvkJBsG