iris: Don't leak the resource for unsupported modifier

Make sure the res struct is free'd before returning.

Fixes: 2dce0e94 ("iris: Initial commit of a new 'iris' driver for Intel Gen8+ GPUs.")
Reviewed-by: Sagar Ghuge <sagar.ghuge@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
(cherry picked from commit f2fc5dec)