drm/i915: Support render nodes

Enable support for drm render nodes for i915 by flagging the ioctls that
are safe and just needed for rendering.

v2: mark reg_read, set_caching and get_caching (ickle, danvet)

Signed-off-by: Kristian Høgsberg <krh@bitplanet.net>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>