drm/vmwgfx: move ttm_bo_swapout_all into vmwgfx

It is the sole user of this.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/393498/