panfrost/ci: Disable GLES2 jobs when we run GLES3

panfrost/ci: Disable GLES2 jobs when we run GLES3

I've been meaning to make this change for a while, but I've been holding
out in case dEQP-GLES2 on T860/Bifrost catches a driver issue missed by
dEQP-GLES3 and also missed by T720's dEQP-GLES2 job. Given that hasn't
happened (indeed, dEQP-GLES3 is an effective super set of dEQP-GLES2),
let's save the cycles and skip the redundant jobs.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10923>