d3d12: Fixes unused-variable compile error

d3d12: Fixes unused-variable compile error

The compile error message is:
../../src/gallium/drivers/d3d12/d3d12_video_screen.cpp:481:70: error: unused variable ‘sliceData’ [-Werror=unused-variable]
  481 |    D3D12_VIDEO_ENCODER_PICTURE_CONTROL_SUBREGIONS_LAYOUT_DATA_SLICES sliceData = { };
      |                                                                      ^~~~~~~~~
cc1plus: all warnings being treated as errors

Signed-off-by: Yonggang Luo <luoyonggang@gmail.com>
Reviewed-by: Sil Vilerino <sivileri@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23900>