gpu: host1x: Fix dma_free_wc() argument in the error path

If IOVA allocation or IOMMU mapping fails, dma_free_wc() is invoked with
size=0 because of a typo, that triggers "kernel BUG at mm/vmalloc.c:124!".

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Reviewed-by: Mikko Perttunen <mperttunen@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>