r600g: propogate resource usage flags to winsys, use to choose bo domains

r600g: propogate resource usage flags to winsys, use to choose bo domains

This opens the question of what interface the winsys layer should
really have for talking about these concepts.

For now I'm using the existing gallium resource usage concept, but
there is no reason not use terms closer to what the hardware
understands - eg. the domains themselves.