meson: Don't compile pipe loader with dri support when not using dri

Corrects building glx as gallium-xlib without any dri targets.

v2: - fix ugly formatting

Fixes: 66c94b93
       ("meson: build gallium winsys for dri, null, and wrapper")

Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
(cherry picked from commit 5dcb77e4)