mdp3: Prevent deref of invalid pointer that contains err code

It is possible that mdp3_iommu_meta_put with a non null iommu_meta
that contains PTR_ERR value.

Change-Id: I440c78e324f558fec22d2db373dfd2c0e4d5fbb7
Signed-off-by: Terence Hampson <thampson@codeaurora.org>