drm/ttm: fix ttm_execbuf_util.h kernel-doc warnings

Fix some formatting errors and excess documentation.

Cc: Christian Koenig <christian.koenig@amd.com>
Cc: Huang Rui <ray.huang@amd.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/b0f6d243c4e5eb1564ef2f4bb5bb834ee2c0305b.1709898638.git.jani.nikula@intel.com