drm/ttm: move initializing ttm->sg into ttm_tt_init_fields

Better to set this with all other fields as well.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Roger He <Hongbo.He@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>