panfrost/ci: Mark draw_buffers_indexed.* as flakes

These keep flaking. Icecream95 observes the issue relates to AFBC in the
discussion of the flake in issue 6604. Until the root cause can be identified
and fixed, mark the tests as known flakes for CI.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Reviewed-by: Emma Anholt <emma@anholt.net>
Part-of: <!16855>
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