i915: Make use of sarea_priv conditional.

We fail ioctls that depend on the sarea_priv with EINVAL.

Signed-off-by: Kristian Høgsberg <krh@redhat.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>