iris: Defer closing and freeing VMA until buffers are idle.

iris: Defer closing and freeing VMA until buffers are idle.

There will unfortunately be circumstances where we cannot re-use a
virtual memory address until it's no longer active on the GPU.  To
facilitate this, we instead move BOs to a "dead" list, and defer
closing them and returning their VMA until they are idle.  We
periodically sweep these away in cleanup_bo_cache, which triggers
every time a new object's refcount hits zero.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Tested-by: Jordan Justen <jordan.l.justen@intel.com>