drm/amdgpu: disable ring_muxer if mcbp is off

Using the ring_muxer without preemption adds overhead for no
reason since mcbp cannot be triggered.

Moving back to a single queue in this case also helps when
high priority app are used: in this case the gpu_scheduler
priority handling will work as expected - much better than
ring_muxer with its 2 independant schedulers competing for
the same hardware queue.

This change requires moving amdgpu_device_set_mcbp above
amdgpu_device_ip_early_init because we use adev->gfx.mcbp.

Signed-off-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Acked-by: Jiadong Zhu <Jiadong.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>