drm/amdgpu: fix sdma v4 startup under SRIOV

Under SRIOV we were enabling the ring buffer before it was initialized.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>