drm/ttm: fix some minor kerneldoc issues

Pointed out by the kernel test robot while merging ttm_bo_api.h and
ttm_bo_driver.h.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230117123345.387078-1-christian.koenig@amd.com