iris: Directly access BOs rather than using iris_resource_bo(...)

iris_resource_bo() is convenient when we only have a pipe_resource * variable, and don't need to do a lot with it other than get at the BO.

When we need to do more with a resource, we usually cast it to iris_resource *, at which point we can just use res->bo instead.

This patch updates iris_copy_region to use src_res->bo, dst_res->bo, rather than iris_resource_bo(src) and iris_resource_bo(dst), since we already have those cast versions on hand.