r600: fix warnings for missing-braces on clang

r600: fix warnings for missing-braces on clang

[2624/3754] Compiling C++ object src/gallium/drivers/r600/sfn/tests/libr600_test.a.p/sfn_test_shaders.cpp.o
../src/gallium/drivers/r600/sfn/tests/sfn_test_shaders.cpp:3066:27: warning: suggest braces around initialization of subobject [-Wmissing-braces]
   r600_shader_key key = {0};
                          ^
                          {}
1 warning generated.
[2711/3754] Compiling C++ object src/gallium/drivers/r600/libr600.a.p/sfn_sfn_nir_lower_64bit.cpp.o
../src/gallium/drivers/r600/sfn/sfn_nir_lower_64bit.cpp:910:33: warning: suggest braces around initialization of subobject [-Wmissing-braces]
      nir_const_value val[4] = {0};
                                ^
                                {}
../src/gallium/drivers/r600/sfn/sfn_nir_lower_64bit.cpp:1328:38: warning: suggest braces around initialization of subobject [-Wmissing-braces]
      nir_ssa_scalar channels[4] = { 0 };
                                     ^
                                     {}

Reviewed-by: Gert Wollny <gert.wollny@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18307>