drm/amdgpu: fix mec queue policy on single MEC asics

Fixes hangs on single MEC asics.

Fixes: 2ed286fb434 (drm/amdgpu: new queue policy, take first 2 queues of each pipe v2)
Reviewed-by: Alex Xie <AlexBin.Xie@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>