d3d12: Fixes compile with MSVC
MSVC compiling error:
nir/nir_lower_shader_calls.c
../mesa/src/compiler/nir/nir_lower_shader_calls.c(1188): error C2220: the following warning is treated as an error
../mesa/src/compiler/nir/nir_lower_shader_calls.c(1188): warning C4101: '_': unreferenced local variable
../mesa/src/compiler/nir/nir_lower_shader_calls.c(1190): warning C4101: '_': unreferenced local variable