ci: Move freedreno's parallelism to the runner instead of gitlab-ci jobs.
I set the runners to concurrency=1, so they serve only one gitlab-ci job at at time. Swap over to using the parallel runner now to keep the runners busy, more efficiently than spawning many docker containers and downloading artifacts multiple times, and producing easier-to-understand results for browsing on the web.
This bumps the a306 runners to 4x parallel instead of 2x like before, but cheza gles3 drops from 6 to 4. Current rough timings of the jobs (if no container download):
db410c-gles2: 5:00 a630-gles2: 1:30 a630-gles3: 6:00 a630-gles31: 5:30
a630-gles3 is a bit longer than I like, but it should come back down once I can sort out the NIR algebraic rewinding.