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.

Change-Id: Ifb400c5dd2cb859c43a0fe712298acaa6a663fec
Signed-off-by: Arun Menon <menon@codeaurora.org>