drm/ttm: fix unreachable code.

None of the in-tree drivers use user objects yet so this wasn't hitting
us.

Stanse found unreachable code in ttm_bo_add_ttm:
http://decibel.fi.muni.cz/~xslaby/stanse/error.cgi?db=32&id=714#l238



Reported-by: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>