drm/radeon: Only create additional ring debugfs files on Cayman or newer.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=46274

Tested with a Cayman card in a Llano system: The additional files are created
and working for the Cayman card but not created for the CPU's built-in GPU.

Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>