drm/amdgpu/sdma4: set wptr shadow atomically (v2)

No functional change until wptr polling uses this
location (future patch).

v2: use WRITE_ONCE

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