kernel-drm: reduce hash order for ttm object and ttm reference object

kernel-drm: reduce hash order for ttm object and ttm reference object

BZ: 31145

TTM is mainly used by video driver, we don't allocate too much ttm buffer object, so reduce hash order, which can save memory,
also avoid too fragmented vmalloc memory.

Change-Id: I102cb26000b5e95a923b2b66920dca83b1274ffc
Reviewed-on: http://android.intel.com:8080/47159
Reviewed-by: Jiang, Fei <fei.jiang@intel.com>
Reviewed-by: Zhang, Yanmin <yanmin.zhang@intel.com>
Reviewed-by: Tu, Yun <yun.tu@intel.com>
Reviewed-by: Zeng, Li <li.zeng@intel.com>
Reviewed-by: Ding, Haitao <haitao.ding@intel.com>
Tested-by: Ding, Haitao <haitao.ding@intel.com>
Reviewed-by: buildbot <buildbot@intel.com>
Tested-by: buildbot <buildbot@intel.com>