msm: vidc: Unmap iommu in non-atomic context

ion_unmap_iommu should not be called in atomic context.
This change removes spinlocks around this call. spinlock is
needed to protect the addition and deletion into the
queues and it is not required while unmapping/freeing
memory.

Change-Id: I8a35f8d9c18302ce4761eda25f4238b66e948430
Signed-off-by: Vinay Kalia <vkalia@codeaurora.org>