drm/radeon: make some asic pointers per ring

Emitting fences, semaphores and ib works differently
on different ring, so its is easier to maintain
separate functions for each ring.

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