drm/ttm: Unexport ttm_global_swapout()

Unexport ttm_global_swapout() since it is not used outside of TTM.

Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230307144621.10748-5-thomas.hellstrom@linux.intel.com