drm/etnaviv: check for reaped mapping in etnaviv_iommu_unmap_gem

In the Linux kernel, the following vulnerability has been resolved: drm/etnaviv: check for reaped mapping in etnaviv_iommu_unmap_gem When the mapping is already reaped the unmap must be a no-op, as we would otherwise try to remove the mapping twice, corrupting the involved data structures.