drm/amdgpu: Add a read to GFX v9.4.3 ring test

Issue a read to confirm the register write before ringing doorbell. With
multiple XCCs there is chance for race condition.

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Acked-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Asad Kamal <asad.kamal@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>