iris: close screen fd on iris_destroy_screen

Otherwise it never gets closed, this fixes errors seen with deqp-egl
where we end up opening 1024 files.

Fixes: 2dce0e94 ("iris: Initial commit of a new 'iris' driver for Intel Gen8+ GPUs.")
Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
(cherry picked from commit 63125538)