drm/amdgpu: enabled software IH ring for Vega

Seems like we won't get the hardware IH1/2 rings on Vega20 working.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>