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

Documentation/gpu/drm-mm.rst

@@ -43,6 +43,12 @@ TTM device object reference
 .. kernel-doc:: drivers/gpu/drm/ttm/ttm_device.c
    :export:
 
+TTM resource placement reference
+--------------------------------
+
+.. kernel-doc:: include/drm/ttm/ttm_placement.h
+   :internal:
+
 TTM resource object reference
 -----------------------------
 

include/drm/ttm/ttm_placement.h

@@ -58,6 +58,7 @@
  *
  * @fpfn:	first valid page frame number to put the object
  * @lpfn:	last valid page frame number to put the object
+ * @mem_type:	One of TTM_PL_* where the resource should be allocated from.
  * @flags:	memory domain and caching flags for the object
  *
  * Structure indicating a possible place to put an object.