intel: Reorder need_fence vs fenced_command to avoid fences on gen4

gen4+ hardware doesn't use fences for GPU access and the older kernel
doesn't expect userspace to make such a mistake. So don't.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=32190


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>