drm/ttm: Remove mm init error printouts and checks

Replace with BUG_ON(). These error messages remained from the time
when TTM was initialized from user-space. Nowadays hitting one of those
is really a kernel bug.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>