ci/amd: Rebalance radeonsi-stoney-gl:x86_64

It is taking 19 min on average to run this job.
As we don't have more capacity, introduce a fraction of 2 and create the
`full` version to be run on nightly pipelines.

Signed-off-by: Guilherme Gallo <guilherme.gallo@collabora.com>
Part-of: <mesa/mesa!31414>