drm/radeon: make forcing ring activity a common function

Nothing chipset or ring specific with it,
so also move it to radon_ring.

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