staging: ion: ion_cma_heap: remove ion_cma_get_sgtable

Remove the temporary code ion_cma_get_sgtable,
use dma_common_get_sgtable instead

Signed-off-by: Zeng Tao <prime.zeng@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>