dmaengine: qcom: reduce vmalloc memory size

SMMU stage 1 is in passthrough mode for GPI clients.
Reduce vmalloc memory to the minimum size when
creating the iommu mapping.

CRs-Fixed: 1970703
Change-Id: I6711c11b487d954af9da10f28f99fd26ad303def
Signed-off-by: Tony Truong <truong@codeaurora.org>