gallium/targets/libgl-gdi: prefer d3d12 driver

Unlike the other drivers, the D3D12 driver is hardware accelerated, so
it's going to be a more reasonable choice. So let's prefer it.

This only matters for people who build with the D3D12 driver. And they
can set the GALLIUM_DRIVER environment variable as appropriate to
override it.

Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Acked-by: Adam Jackson <ajax@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7534>

src/gallium/targets/libgl-gdi/libgl_gdi.c

@@ -89,7 +89,9 @@ gdi_screen_create(HDC hDC)
    if(!winsys)
       goto no_winsys;
 
-#ifdef GALLIUM_LLVMPIPE
+#ifdef GALLIUM_D3D12
+   default_driver = "d3d12";
+#elif defined(GALLIUM_LLVMPIPE)
    default_driver = "llvmpipe";
 #elif GALLIUM_SWR
    default_driver = "swr";