scsi: hisi_sas: Use dmam_alloc_coherent()

This patch replaces the usage of dma_alloc_coherent() with the managed
version, dmam_alloc_coherent(), hereby reducing replicated code.

Signed-off-by: Xiang Chen <chenxiang66@hisilicon.com>
Signed-off-by; John Garry <john.garry@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>