meson,ci: misc fixes following the swrast split into llvmpipe & softpipe

meson: don't select the deprecated `swrast` option ourselves

Users get the deprecation warning but didn't do anything, they left
things to `auto` and we pick the deprecated `swrast`? Hardly seems fair!

(I forgot to do this when I added the deprecation warning to ajax's commit)

Fixes: 010b2f9497ab256d9e80 ("gallium/meson: Deconflate swrast/softpipe/llvmpipe")

meson: improve wording of "incompatible llvm options" error

ci: remove llvmpipe in the job that disables llvm
Instead of removing it from all the arm build jobs and only adding it
back on arm64.

Fixes: 35cb0c350e6e682d9746 ("ci: replace gallium-drivers=swrast with gallium-drivers=llvmpipe,softpipe")