drm/amdgpu: enable WPTR_POLL_ENABLE for sriov on sdma_v6_0

[Why]
Under SR-IOV, if VF is switched out then its doorbell will be disabled,
SDMA rely on WPTR_POLL to get doorbells which was sent during VF
switched-out time.

[How]
For SR-IOV, set SDMA WPTR_POLL_ENABLE to 1.

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Horace Chen <horace.chen@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>