drm/amdgpu: enable IH doorbell for ring 1&2 on Vega

The doorbells should already be reserved, just enable them.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Chunming Zhou <david1.zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>