msm: vidc: error handling for ion_map_iommu calls.

Added a check for non-zero virtual address.
Without this change, when ion_map_iommu returns a
zero virtual address, video core continuous ahead
without error check and later target reset happens
when trying to do a ion_unmap_iommu on the above fd,
which was never mapped.

Signed-off-by: Arun Menon <menon@codeaurora.org>
(cherry picked from commit 8688368ce3afbd1f3a80c7fa688c6a2f467247d3)

Change-Id: Ic6d86cc621e2f524dd19d1677646822cdc0f317c
Signed-off-by: Sudhir Sharma <sudsha@codeaurora.org>