drm/amdgpu: Disable SDMA WPTR_POLL_ENABLE for sdma_v6_0

WPTR_POLL_ENABLE = 1 was kept to support legacy doorbell programming in
SimNow environment. Disable for real hardware.

Signed-off-by: Graham Sider <Graham.Sider@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>