drm/ttm: fix ttm_bo_wait

reservation_object_wait_timeout_rcu() should enable signaling even with a
zero timeout, but ttm_bo_wait() can also be called from atomic context and
then it is not a good idea to do this.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
  [sumits: fix checkpatch warnings]
Link: http://patchwork.freedesktop.org/patch/msgid/1478553376-18575-3-git-send-email-alexander.deucher@amd.com