drm/ttm: Export ttm_bo_get_unless_zero()

Export ttm_bo_get_unless_zero() to be used when looking up buffer
objects that are removed from the lookup structure in the destructor.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>
Reviewed-by: Christian König <christian.koenig@amd.com>