dmaengine: gpi: remove sg ring since it's not used

We're not taking advantage of sg ring in gpi driver,
remove the ring to reduce event processing time.

CRs-Fixed: 2165975
Change-Id: Id47a28cf241834268afcc25c919bd8127acb8511
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>