ci/freedreno: Cut down pre-merge a630 VK coverage.
We've got lots of VK coverage on 618, so take some of the load off (but leave a little bit of testing just to make sure we don't totally break 630). This should help with our Marge times since we've added some other coverage to 618 that's started overloading the runners.
(and a couple other a630 CI touchups here, too)