drm/amdgpu/sdma4: drop allocation of poll_mem_offs

We already allocate this as part of the ring structure,
use that instead.

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