r600g: add support for indirect geom ring writes

r600g: add support for indirect geom ring writes

We need to be able to write to the ring using a base register
for when we emit vertices in a loop, in theory the SB compiler
could collapse these indirect writes to direct writes if the
register value is constant and known, but that is outside my
pay grade.

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