dmaengine: vdma: Use dma_pool_zalloc

dma_pool_zalloc combines dma_pool_alloc and memset 0
this patch updates the driver to use dma_pool_zalloc.

Signed-off-by: Kedareswara rao Appana <appanad@xilinx.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>