pvr: do not use fallthrough for unreachable code

pvr: do not use fallthrough for unreachable code

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>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15531>