drm/ttm: enable TTM placement kerneldoc

Fix the last remaining warning and finally enable this.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210908132933.3269-7-christian.koenig@amd.com