egl: don't expose swrast device if swrast is not built This fixes piglit/egl_ext_device_base without swrast. Reviewed-by: Yogesh Mohan Marimuthu <yogesh.mohanmarimuthu@amd.com> (cherry picked from commit 48e218b49d6bdfd167b755e3983bce4f14132347) Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/28885>