drm/amdgpu/gfx: keep all compute queues on the same pipe

Spreading them causes performance regressions using compute
queues on Polaris 11.

Cc: Jim Qu <jim.qu@amd.com>
Acked-by: Jim Qu <Jim.Qu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>