drm/amdgpu/gfx9: fix compute ring doorbell index

This got lost when the code was revamped.  Copy/paste bug from
gfx8.

Reported-by: Evan Quan <evan.quan@amd.com>
Fixes: 78c16834 (drm/amdgpu: allow split of queues with kfd at queue granularity v4)
Reviewed-by: Junwei Zhang <Jerry.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>