drm/ttm, drm/vmwgfx: move cpu_writers handling into vmwgfx

This feature is only used by vmwgfx and superfluous for everybody else.

Signed-off-by: Christian König <christian.koenig@amd.com>
Co-developed-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Tested-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/333650/