drm/radeon: support render nodes

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

Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>