drm/ttm: introduce ttm_bo_move_null

This pattern is cut-n-pasted across 4 drivers, switch it to
a WARN_ON instead, as BUG_ON is considered a bad idea usually.

Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200907204630.1406528-2-airlied@gmail.com