unreachable() doesn't lead to executing the code that follows it,
neither in debug nor release builds. So falling through doesn't make any
sense.
This fixes a compile-error on clang.
Let's move the default-block to the end to make it clearer that there's
no intended fallthrough.
Reviewed-by: Frank Binns <frank.binns@imgtec.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15531>