gpu: host1x: Fix potential double free if IOMMU is disabled

If context device has no IOMMU, the 'cdl->devs' is freed in
error path, but host1x_memory_context_list_init() doesn't
return an error code, so the module can be loaded successfully,
when it's unloading, the host1x_memory_context_list_free() is
called in host1x_remove(), it will cause double free. Set the
'cdl->devs' to NULL after freeing it to avoid double free.

Fixes: 8aa5bcb6 ("gpu: host1x: Add context device management code")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Mikko Perttunen <mperttunen@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>