drm/ttm: Fix memory leak in ttm_agp_backend.c

This patch fix a memory leak found by cppcheck.
[drivers/gpu/drm/ttm/ttm_agp_backend.c:129]:
(error) Memory leak: agp_be

Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>