drm/amdgpu/sdma3: correct wptr initialization for sdma

If doorbell is used for wptr update, we also need to use it
to initialize wptr to 0.

Signed-off-by: Yintian Tao <yttao@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>