Improve TTMs empty object handling

Hi everyone,

re-sending this because Daniel was requesting a background why this is useful.

When TTM creates a buffer this object initially should not have any backing store and there no resource object associated with it. The same can happen when a driver requests that the backing store of an object is destroyed without allocating a new one.

This is really useful during initial buffer creation as well as temporary buffers and page tables which content doesn't need to be preserved when they are evicted.

Currently TTM allocates dummy system resources for that because drivers couldn't handle a NULL pointer there. Audit the drivers and then clean up TTM to stop making those dummy allocations.

Please review and comment, Christian.