drm/amdgpu: enabled software IH ring for Navi

Felix pointed out that we need this for Navi as well.

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>