drm/amdgpu: enable software ih ring for vega20 ih block

software ih ring will be used as a workaround
in case hardware ih ring 1 and ring 2 don't work

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Feifei Xu <Feifei.Xu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>