drm/amdgpu: disable SDMA WPTR_POLL_ENABLE for SR-IOV

[Why]
This WPTR_POLL_ENABLE is a hardware contigious polling which will cause
FCLK and UCLK to keep on a high level.
Mostly its case can be covered by F32_WPTR_POLL_ENABLE which polls by
firmware.
So to save power, SR-IOV also needs to disable this bit

Signed-off-by: Horace Chen <horace.chen@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>