drm/amdgpu: add ring test for page queue

We add page queue for sdma to update page table. So here it also needs ring test
to verify it workable during the initialization.

Signed-off-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>