msm: vidc: Fix for IOMMU mapping.

Same memory region was being mapped multiple times.
This fix avoids this scenario and makes sure that
that we map proper size and single fd is mapped
and unmapped one time only.

Change-Id: Ibba932456792d5964213fe93db2c6101ad94b74c
Signed-off-by: Vinay Kalia <vkalia@codeaurora.org>