gitlab-ci: rules:changes to test on tested drivers changes

gitlab-ci: rules:changes to test on tested drivers changes

For now tests only use these drivers:
  * llvmpipe
  * softpipe
  * freedreno
  * lima
  * etnaviv
  * panfrost

So using rules:changes gitlab feature to run the tests when the changes
made are potentially affecting these drivers.

A few notes:
  * the following code:
      .piglit-test:
        extends:
          - .test-gl
          - .llvmpipe-rules
    makes gitlab replace .test-gl "rules:changes" values by the one from
    ".llvmpipe-rules".
  * rules:changes always matches for non-MR new branches so jobs will always be
    created (and they'll be run if their dependencies are run). For pushes to
    existing branches the files changed by the push are used to match the
    rules:changes path.
  * the same gitlab feature could be used for some build jobs

Acked-by: Eric Engestrom <eric@engestrom.ch>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/2569>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/2569>