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.

Cc @tomeu