iris: Fix import sync-file into syncobj

iris: Fix import sync-file into syncobj

When importing a sync-file, the kernel expects to be told which syncobj
to replace with the new fence -- it does not automatically create a new
handle for us. Abide by this rule and create a new syncobj for the
imported sync-file.

Fixes: f459c56be6bf ("iris: Add fence support using drm_syncobj")
Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3919>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3919>