Clear batch buffer pointer after unmap

Clear batch buffer pointer after unmap

Fix libdrm warning about unmap the unmapped buffer, as when
batch terminate we will try to do unmap again, but batch has
already been unmapped.

Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Reviewed-by: Zhigang Gong <zhigang.gong@linux.intel.com>