This decouples Kopper and swrast. With this change, dri/kopper.c no
longer references DRIswrastLoaderEsxtension at all.
Reviewed-by: Adam Jackson <ajax@redhat.com>
Reviewed-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/36014>