egl: don't bind the wayland display if the extension is not available

When running in a purely swrast environment (such as with
LIBGL_ALWAYS_SOFTWARE), the extension is not exposed by mesa,
but wayland is still possible with wl_shm.

https://bugzilla.gnome.org/show_bug.cgi?id=704750

Reviewed-by: Neil Roberts <neil@linux.intel.com>