gitlab-ci: merge several meson jobs

gitlab-ci: merge several meson jobs

Merge the following into `meson-main`/`meson-loader-classic-dri`/
`meson-gallium-swr`:
- meson-vulkan
- meson-gallium-drivers-other
- meson-gallium-st-other

Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
[ Michel Dänzer ]
* Rebase and fix up commit log.
* Don't set VULKAN_DRIVERS in meson-loader-classic-dri.
* Remove extraneous whitespace.
* Squash in follow-up fixes.

Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
[ anholt]
* Add a note why nine and swrast landed where they did.
* Switch from s/meson-vulkan/meson-main/ to
  s/meson-loader-classic-dri/meson-main/ which I think was the original
  intent

Reviewed-by: Eric Anholt <eric@anholt.net>
Acked-by: Eric Engestrom <eric.engestrom@intel.com> (anholt changes)
Acked-by: Dylan Baker <dylan@pnwbakers.com>