drm/ttm: make unlocking in ttm_bo_cleanup_refs optional v3

Needed for the next patch.

v2: actually predicate all unlocks
v3: add some cleanups suggested by Michel.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-and-Tested-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>