drm/amdgpu/sdma4: use paging queue for buffer funcs

Use the paging queue for buffer functions to avoid contention
with the other queues.

Reviewed-by: Junwei Zhang <Jerry.Zhang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Tested-by: Chen Gong <Curry.Gong@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>