scsi: hisi_sas: optimise DMA slot memory

Currently we allocate 3 sets of DMA memories from separate pools for
each slot. This is inefficient in terms of memory usage
(buffers are less than 1 page in size, so we lose due to alignment),
and also time spent in doing 3 allocations + de-allocations per slot,
instead of 1.

To optimise, combine the 3 DMA buffers into a single buffer from a
single pool.

Signed-off-by: Xiaofei Tan <tanxiaofei@huawei.com>
Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>