iris/bufmgr: Stop changing mapping modes on buffers

With discrete GPUs, we aren't able to change mapping modes after we've created a buffer. This is a limitation of TTM. However, we already have a buffer cache and it's pretty likely that stuff in any given memzone will end up with just the one mapping type anyway so this shouldn't have much of a cost.

Acked-by: Daniel Vetter daniel.vetter@ffwll.ch