panfrost/ci: Correctly set DEQP_FRACTION for LAVA

I was under the impression baremetal and LAVA had unified their variable
handling, apparently that was assigned to marge but closed in the end.
So this variable was being ignored and T860-GLES31 is taking way too
long. Fix that.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
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