r600/sfn: allow building with clang 6 (Android 9)

r600/sfn: allow building with clang 6 (Android 9)

static constexpr const 'value' is replaced by static function
in all type_char template specializations
to avoid the following building errors happening with clang 6

/home/utente/pie-x86_kernel/prebuilts/clang/host/linux-x86/clang-4691093/bin/ld.lld: error: undefined symbol: r600::type_char<r600::ExportInstr>::value
>>> referenced by sfn_scheduler.cpp
>>>               sfn_sfn_scheduler.cpp.o:(bool r600::BlockSheduler::collect_ready_type<r600::ExportInstr>(std::__1::list<r600::ExportInstr*, std::__1::allocator<r600::ExportInstr*> >&, std::__1::list<r600::ExportInstr*, std::__1::allocator<r600::ExportInstr*> >&)) in archive src/gallium/drivers/r600/libr600.a
...
/home/utente/pie-x86_kernel/prebuilts/clang/host/linux-x86/clang-4691093/bin/ld.lld: error: undefined symbol: r600::type_char<r600::RatInstr>::value
>>> referenced by sfn_scheduler.cpp
>>>               sfn_sfn_scheduler.cpp.o:(bool r600::BlockSheduler::collect_ready_type<r600::RatInstr>(std::__1::list<r600::RatInstr*, std::__1::allocator<r600::RatInstr*> >&, std::__1::list<r600::RatInstr*, std::__1::allocator<r600::RatInstr*> >&)) in archive src/gallium/drivers/r600/libr600.a
clang-6.0: error: linker command failed with exit code 1 (use -v to see invocation)

Cc: "22.2" "22.3" mesa-stable
Reviewed-by: Gert Wollny <gert.wollny@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19873>
(cherry picked from commit e74d989a6935ce11d06970a3c98b474b7773c905)