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>