wayland: Only init EGLStream controller if we didn't end up with gbm

When we use gbm together with the NVIDIA driver, we want the EGL/Vulkan
clients to do the same, instead of using the EGLStream paths. To achieve
that, make sure to only initialize the EGLStream controller when we
didn't end up using gbm as the renderer backend.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2052>
(cherry picked from commit ac907119)