drm/rockchip: don't leak iommu mapping

arm_iommu_attach_device() takes its own reference to the mapping we give
it.  Since we do not keep a reference to the mapping ourselves, we must
release it before returning.

Also fix the error path, which fails to release the mapping if it has
called arm_iommu_detach_device() since that clears archdata.mapping.

Signed-off-by: John Keeping <john@metanate.com>