drm/ttm: make sure format string cannot leak in

While zone->name is currently hard coded, the call to kobject_init_and_add()
should follow the more defensive argument list usage (as already done in
other places in ttm_memory.c) where "%s" is used instead of directly passing
in a variable as a format string.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>