Fixes doorbell reflection on Vega20.

Change-Id: I0495139d160a9032dff5977289b1eec11c16f781
Signed-off-by: Jay Cornwall <Jay.Cornwall@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>