drm/ttm: Export the ttm_k[un]map_atomic_prot API.

It will be used by vmwgfx cpu blit.

Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>