drm/amdgpu: rework scheduler submission handling.

Remove active_hw_rq and it's protecting queue_lock, they are unused.

User 32bit atomic for hw_rq_count, 64bits for counting to three is a bit
overkill.

Cleanup the function name and remove incorrect comments.

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