drm/ttm: partial revert "cleanup ttm_tt_(unbind|destroy)" v3

We still need to unbind explicitly during a move.

This partial reverts commit ff20caa0bcbfef9f7686f8d1868a3b990921afd6.

v2: remove unnecessary check and unused variable
v3: fix typo in commit message

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>