CI of freedreno

This MR introduces CI of the freedreno driver on a306 and a630 on future MRs, as a step toward implementing the "Not Rocket Science Rule Of Software Engineering".

This new MR replaces !1561 (closed) -- the "ci-fd" branch name as the merge source there meant that every new push results in the tests being executed twice -- once for anholt/mesa due to the "ci-" prefix, and once for the MR. Flipping the name around will reduce CI overhead as I work on finishing this up.

Also note the new .gitlab-ci/README.md in this version.

TODO (not necessarily before merge):

• Bring up more than 3 db410cs to bring down runtime
  • split 6 ways, GLES2 jobs are ~2.5 minutes each at concurrent=1, 4 minutes at concurrent=2
  • currently have 3 runners at concurrent=2
  • Once we have more, we can probably turn GLES3 on.
• Do a bunch more builds to make sure our cheza test results are reliable.
• Test containers are 624M currently, can we shrink them?
• Document farm setup better for reproducing with other HW
  • db410c setup info here
  • cheza setup info here
  • I'm setting up my runners to be shared to make it pre-merge, but anyone could do something like this with their own HW on their own Mesa with private runners for test automation.
• Deduplicate cross container's VK-GL-CTS build with llvmpipe CI's (can we do this with the ci-templates?)