r600/sfn: Fix test compilation with -fno-rtti
79ca456b reintroduced the use of dynamic_cast<> in r600/sfn tests. This breaks compilation with -fno-rtti, as required to build against the LLVM configuration recommended upstream. Use static_cast<> instead to fix this.
Fixes: #7820 (closed) Signed-off-by: Michał Górny mgorny@gentoo.org