adsprpc: Initialize VA of fastrpc dynamic heap buffer to Zero

As fastrpc dynamic heap buffer is allocated with DMA_ATTR_NO_KERNEL_MAPPING
there is no virtual mapping for this buffer, so initialize VA with Zero.

Change-Id: Id0fd62baa2a9a6510b909bdec69d02d0e4c8f9ab
Acked-by: Chenna Kesava Raju <chennak@qti.qualcomm.com>
Signed-off-by: Tharun Kumar Merugu <mtharu@codeaurora.org>