drm/amdgpu: add amd_sched_next_queued_seq function

This function is used to get the next queued sequence number

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