gallium/radeon: derive buffer placement and flags only at initialization

gallium/radeon: derive buffer placement and flags only at initialization

Invalidated buffers don't have to go through it.

Split r600_init_resource into r600_init_resource_fields and
r600_alloc_resource.

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>