CI: Disable Panfrost T720/T760

CI: Disable Panfrost T720/T760

We're reconfiguring our Cambridge office lab networking and physical
setup for more scalability amongst other things. We can still run jobs
on one RK3399 at the peak outage, but we'll lose the T7x0 this morning,
so disable it until it's all back.

T820 is still disabled due to an unrelated BayLibre internal outage.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5296>