Gap in DeferredContext support for Map(DISCARD) on a buffer

[vdwtanner]

There is one known gap with D3D11On12’s deferred context support, which is if someone tries to create an ID3D11CommandList that has a Map(DISCARD) operation on a buffer, and then execute that command list more than once.

https://github.com/microsoft/D3D11On12/blob/master/src/device.cpp#L955