drm/radeon/kms: add support for CP setup on cayman asics

Cayman asics have 3 ring buffers:
ring 0 supports both gfx and compute
rings 1 and 2 are compute only

At the moment we only support ring 0.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>