fix -Wmissing-braces warnings on clang
Fixes the following warnings with 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 };
^
{}
2 warnings generated.
[3485/3754] Compiling C++ object src/gallium/drivers/d3d12/libd3d12.a.p/d3d12_screen.cpp.o
../src/gallium/drivers/d3d12/d3d12_screen.cpp:56:65: warning: suggest braces around initialization of subobject [-Wmissing-braces]
static GUID OpenGLOn12CreatorID = { 0x6bb3cd34, 0x0d19, 0x45ab, 0x97, 0xed, 0xd7, 0x20, 0xba, 0x3d, 0xfc, 0x80 };
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
{ }