drm/ttm: enable TTM device object kerneldoc v2

Fix the remaining warnings, switch to inline structure documentation
and finally enable this.

v2: adjust based on suggestions from Alex

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210908132933.3269-5-christian.koenig@amd.com