CI: Disable Panfrost T860 and AMD Stoney tests

Due to scheduled physical lab maintenance to prepare for expansion and
better shard our device types across redundant infrastructure, these
device types will be inaccessible for approx. 3 hours. Disable them
until they are back.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6302>
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