kernel-drm: remove ttm_bo_reserve for TTM_PL_SYNCCPU_OP_RELEASE ioctl.

kernel-drm: remove ttm_bo_reserve for TTM_PL_SYNCCPU_OP_RELEASE ioctl.

BZ: 34332

Some time ttm_bo_reserver will fail, then will not decrease cpu_writers,
while pool_releasefromcpu and wsbmBOReleaseFromCpu will not handle the return error.
ttm_ref_object_base_unref will only access the bo->write_counter with atomic_dec_and_test, which is atomic operation.

Change-Id: Ie831fad02699ec1875d73e238edaf3b2ff10f4d9
Reviewed-on: http://android.intel.com:8080/47556
Reviewed-by: Jiang, Fei <fei.jiang@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>