drm/ttm: update kernel-doc line comments

Update kernel-doc line comments to fix warnings reported by make W=1.

drivers/gpu/drm/ttm/ttm_memory.c:271: warning: Function parameter or
member 'glob' not described in 'ttm_shrink'
drivers/gpu/drm/ttm/ttm_memory.c:271: warning: Function parameter or
member 'from_wq' not described in 'ttm_shrink'
drivers/gpu/drm/ttm/ttm_memory.c:271: warning: Function parameter or
member 'extra' not described in 'ttm_shrink'
drivers/gpu/drm/ttm/ttm_memory.c:271: warning: Function parameter or
member 'ctx' not described in 'ttm_shrink'

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Acked-by: Huang Rui <ray.huang@amd.com>
Link: https://patchwork.freedesktop.org/patch/391317/
Signed-off-by: Christian König <christian.koenig@amd.com>

drivers/gpu/drm/ttm/ttm_memory.c

@@ -259,7 +259,7 @@ static bool ttm_zones_above_swap_target(struct ttm_mem_global *glob,
 	return false;
 }
 
-/**
+/*
  * At this point we only support a single shrink callback.
  * Extend this if needed, perhaps using a linked list of callbacks.
  * Note that this function is reentrant: