llvmpipe: lp_resource_copy cleanup

llvmpipe: lp_resource_copy cleanup

We don't need to flush resources for each layer, and since we don't actually
care about layer at all in the flush function just drop the parameter.
Also we can use util_copy_box instead of repeated util_copy_rect.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>