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

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: <!15531>
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