BugLink: http://bugs.launchpad.net/bugs/1546572

The radeon_asic_ring structures are never modified, so declare them as
const.

Done with the help of Coccinelle.
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit d26678da)
Signed-off-by: Alberto Milone <alberto.milone@canonical.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>