drm/amdgpu: Add SPSC queue to scheduler.

It is intended to sabstitute the bounded fifo we are currently
using.

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