drm/radeon: rename struct radeon_cp to radeon_ring

That naming seems to make more sense, since we not
only want to run PM4 rings with it.

Signed-off-by: Christian König <deathsimple@vodafone.de>
Reviewed-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>