drm/amdgpu/sdma3: Enable sdma wptr polling for SRIOV

When hypervisor triggering FLR for one of VFs, need to enable sdma
wptr polling to avoid missing wptr update if enabling doorbell.

Signed-off-by: Xiangliang.Yu <Xiangliang.Yu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>