graphics/mesa-dri: revert r512573 and limit to llvm80 after r512440

Mesa 18.3 doesn't support LLVM 9. While some fixes were backported
there're probably more issues. Apparently, Gnome shows black screen.
As the port is unlikely to be ready for future LLVM_DEFAULT bumps
without a version update just limit to previously tested value.

PR: 239682
Requested by: imp