meson: allow building all glx without any drivers

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111016


Fixes: a47c525f
       ("meson: build glx")
Acked-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
(cherry picked from commit 7cf50af6)