drm/ttm: ttm_tt_init_fields() can be static

Fixes: 75a57669 ("drm/ttm: add ttm_sg_tt_init")
Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.kernel.org/patch/10263323/