drm/i915: don't set up gem ring functions on gen5 for !kms

We already disallow initialition of gem in this case in the
corresponding ioctl, so don't bother setting up the gem support ring
functions in the legacy dri render ring init.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>