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>
2 files changed