drm/amdgpu: use polling mem to set SDMA3 wptr for VF

On Tonga VF, there're 2 sources updating wptr registers for
sdma3: 1) polling mem and 2) doorbell. When doorbell and polling
mem are both enabled on sdma3, there will be collision hit in
occasion between those two sources when ucode and h/w are doing
the updating on wptr register in parallel. Issue doesn't happen
on CP GFX/Compute since CP drops all doorbell writes when VF is
inactive. So enable polling mem and don't use doorbell for SDMA3.

Signed-off-by: Pixel Ding <Pixel.Ding@amd.com>
Reviewed-by: Monk Liu <monk.liu@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>