drm/amdgpu: make sure retry faults are handled in a work item on Vega

Looks like we can't enabled the IH1/IH2 feature for Vega20, make sure
retry faults are handled on a separate ring anyway.

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>