drm/radeon: allow geom rings to be setup on r600/r700 (v2)

the evergreen CS parser has allowed this for a while, just port
the code to the r600 one.

This is required before geom shaders can be made work.

v2: agd5f: minor cleanup and add additional 7xx reg.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>