drm/amdgpu: stop disable the scheduler during HW fini

When we stop the HW for example for GPU reset we should not stop the
front-end scheduler. Otherwise we run into intermediate failures during
command submission.

The scheduler should only be stopped in very few cases:
1. We can't get the hardware working in ring or IB test after a GPU reset.
2. The KIQ scheduler is not used in the front-end and should be disabled during GPU reset.
3. In amdgpu_ring_fini() when the driver unloads.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Nirmoy Das <nirmoy.das@amd.com>
Test-by: Dennis Li <dennis.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>