ci/freedreno: mark a530_piglit as postmerge job
What does this MR do and why?
ci/freedreno: mark a530_piglit as postmerge job
Fixes: dfc4a68b0aad ("ci/freedreno: re-enable piglit tests for Adreno 530 in nightly runs")
Signed-off-by: David Heidelberg <david@ixit.cz>