drm/qxl: Fix return for qxl_release_alloc

This function return handle to allocated release object which is an int.

Signed-off-by: Frediano Ziglio <fziglio@redhat.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>