ci/deqp: set default platform to `default` instead of glx, to also support wayland

In our case (because we have the libs installed), -D DEQP_TARGET=default = xlib (what -D DEQP_TARGET=x11_glx enables) + xcb + wayland

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Part of effort to start running wsi tests in CI; I'm not there yet but no reason not to merge this already :)

(also includes an unrelated minor clean-up, to avoid re-generating the containers multiples times)