drm/amdgpu/sdma4: move wptr polling setup

Move it up before ring enablement with all of the other
engine setup and explicitly disable it for bare metal.

Cc: Frank Min <Frank.Min@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>