drm/radeon/kms: add some new ring params to better handle other ring types

Some rptr/wptrs fields have different offsets and not all rings are pm4
so add a new nop field.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Christian König <deathsimple@vodafone.de>
Signed-off-by: Dave Airlie <airlied@redhat.com>