drm/ttm: reset user pages to WB if modified by ttm

drm/ttm: reset user pages to WB if modified by ttm

BZ: 37816

Set user pages to UC/WC to improve vec performance, need to reset user pages to WB when release TTM buffer object

Change-Id: I110c3dfd1a77d2a60f3d1b0be43563f8c033c7c4
Signed-off-by: Li Zeng <li.zeng@intel.com>
Reviewed-on: http://android.intel.com:8080/52473
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>