ttm: Fix spelling mistakes and remove unused #ifdef

. and some comments to make it easier to understand.

Ackedby: Randy Dunlap <randy.dunlap@oracle.com>
[v2: Added some more updates from Randy Dunlap]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
diff --git a/include/drm/ttm/ttm_memory.h b/include/drm/ttm/ttm_memory.h
index b199170..26c1f78 100644
--- a/include/drm/ttm/ttm_memory.h
+++ b/include/drm/ttm/ttm_memory.h
@@ -41,7 +41,7 @@
  * @do_shrink: The callback function.
  *
  * Arguments to the do_shrink functions are intended to be passed using
- * inheritance. That is, the argument class derives from struct ttm_mem_srink,
+ * inheritance. That is, the argument class derives from struct ttm_mem_shrink,
  * and can be accessed using container_of().
  */