dxil: Fixes warning about comparing different enum value and enum literal

dxil: Fixes warning about comparing different enum value and enum literal

warnings:
../../src/microsoft/spirv_to_dxil/spirv_to_dxil.c: In function 'spirv_to_dxil':
../../src/microsoft/spirv_to_dxil/spirv_to_dxil.c:101:14: warning: comparison between 'dxil_spirv_shader_stage' and 'enum <anonymous>' [-Wenum-compare]
  101 |    if (stage == MESA_SHADER_NONE || stage == MESA_SHADER_KERNEL)
      |              ^~
../../src/microsoft/spirv_to_dxil/spirv_to_dxil.c:101:43: warning: comparison between 'dxil_spirv_shader_stage' and 'enum <anonymous>' [-Wenum-compare]
  101 |    if (stage == MESA_SHADER_NONE || stage == MESA_SHADER_KERNEL)
      |                                           ^~

Signed-off-by: Yonggang Luo <luoyonggang@gmail.com>
Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16671>